Which Mavens?

Are you picky about having left/right bite points feeling the same?  If so, you want the SLV or ULT for the contact adjuster.  In my experience its near impossible to make them match without a functioning contact adjuster (due to pad wear differences etc), regardless of brake brand/model.  The contact adjuster is a big perk to the Mavens IMO.  I haven't tried the Maven Bronze, but I have used a Code R in the past and compared to a Code RSC I found the lack of adjustment drove me nuts.

I've tried both the A1 cams and B1 cams in my Ults, and prefer the A1 as it's more direct feeling and less of a gradual buildup of power.  With A1s I start with the contact adjusters wound all the way in, moving out as pads wear.  With B1s, I started with the contact adjuster most of the way out to compensate for the longer throw that comes with the softer lever feel of them.  I prefer a more direct, sharp brake engagement where the power hits up front, as opposed to brakes that have a softer deadstroke but you have to pull harder to get full power (like Dominions).  I find that lets me shorten my braking zones and get off the brakes more.

Slap a little strip of skateboard/step grip tape on the stock levers to bring back some of the texture you're used to from the XT lever dimples.

For descents that long with an ebike, I'd try to get the A1 calipers (with bigger pistons) over the B1 or Base calipers with smaller pistons.  You can always tune the power level later with rotor size or pad compound.

Shimano lover here. The A1 ultimate or silver is what you want. Super direct and crisp and fantastic lever adjustment with both reach and contact point .

I haven't ridden the B1, but it sounds sounds less shimano-y

Also a Shimano lifer until recently - just switched from XTs to Maven Bases on my daily driver, couldn't be more happy with them! Took exactly 1 ride to get used to the lever feel and massive increase in power, which I think is delivered more smoothly and predictably than even the most perfectly bled Shimano brake. Can't speak to the higher tiers but definitely going to make the switch for my bike park rig next. 

I feel like I can finally add input to this thread.  I have just about every flavor of Maven's in my fleet and I just got the chance to try out the b1.  I have a set of expert a1 (on my nomad 6), bronze a1 (on my ebike) and base that I just converted to b1's with a silver lever assembly (on my ht4 trail bike).  I am also trying out a set of maven base levers with db8 calipers at the moment on a weird ultra light trail bike experiment build... but that's a story for a different thread.  

A few comments that I'll keep in bullet form to keep it succinct:

• I weigh 175lbs and ride an XL.  I run metallic pads and 200mm hs2 rotors on every bike/brake setup, so keep that in mind.  Most of my riding is on very steep shorter trails (2-3 minute, ~1k descents) or 'bigger mountain' stuff that's 5-8 minutes and ~2k.  So keep that in mind with my recommendations. 
• TLDR: I would suggest the b1s to any new maven user as they seem like the goldilocks solution for me.  
• Power/Modulation
  • A1s are incredibly touchy and the power is arguably "too much" at times on my enduro bike and it's easy to lock a wheel unintentionally with a panic grab of the lever.  I never have this sensation on my sc bullit ebike... but the first ride back on my a1s on my enduro bike takes an adjustment or two.  
  • Base are the exact opposite of the A1 in terms of coming on super quick.  They almost have the opposite problem where it's hard to tell when the brake is going to lock because they come on so gently.  Once you adjust to them, the modulation point is really easy to stay on the limit of and the super light lever feel is nice... but maybe removes a bit too much feedback.  I do feel like I am pulling the levers 20-30% harder to initiate maximum possible braking force (maximum just before a lock up), but it's not so significant that it's arm pump inducing and it still feels maybe 2x as powerful as a code.  
  • B1 are sort of the best of both worlds in this regard.  I feel like they have the right amount of softer initial grab/bite and then ramp up appropriately through the lever range/throw and end up at the same lever force at max braking that the a1 do (at least in practical application).  
• Ergonomics - Honestly the ergonomics are the biggest difference between all three.  
  • A1 - Pretty easy to get them dialed exactly how you want them.  They are going to have a pretty short lever throw from pad contact to full engagement.  On silver and above, you can use the contact adjust to change this, but it changes the leverage ratio and ultimately doesn't fully tame how snappy they are.  TLDR: Extremely small dead band, very narrow modulation window, very shimano like.
  • Base - Very difficult to get the right engagement point balanced with the right lever to bar distance at max braking.  No matter what I did, I ended up with a super long lever throw and I would occasionally find myself relaxing my grip to reach for the lever.  In the most extreme circumstances, I had the lever fly out from under my finger.  TLDR: Huge dead band, very large modulation window, difficult to balance lever starting vs ending point and overall lever throw.  Very TRP DHR Evo like. 
  • B1 - Best of all worlds.  There is still a noticeable difference between b1 and a1 for the lever starting point, full lock (in relation to the bar) and the modulation point.  That being said, I think the B1 does strike the best of both worlds where the engagement point is intuitive and the dead band is just right in terms of feel and amount.  TLDR: Very much like a dialed set of code RSC with 2x the power.  Good modulation but not too much, easy to balance full lever throw and position in relation to max braking.
• Which model?
  • I do not really like, nor use contact adjust.  The only time it is nice to have is when a set of pads is brand new as I find that for the first 2-3 rides, the lever to bar distance is a bit more then I want, even with the reach adjusters wound so the lever is as close to the bar as I can get.  
  • The bronze/base lever assembly with the bushings is really not bad... but there is noticeable vertical lever play (perpendicular to the direction of the master cyl).  You don't really feel this a ton while under braking... but when I get back on my expert brakes with the bearings it does feel more direct.  Also, the bushing do tend to squeak a bit as they wear and triflow will quiet it, but not completely.  
  • If you're on a budget, then bronze is completely acceptable and other than play in the lever, I feel like you're giving up nothing.  That being said, for the minimal amount of more expense, the silvers are an obvious choice to get the bearing and the contact adjust.