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I've got a 120, 150, 170 bike, and have a really hard time deciding which bike to ride sometimes (life's tough right?). I have such a hard time because I can ride all the same trails on all my bikes. Choosing a bike that best fits the terrain I ride is near impossible because I can literally ride all my trails on any modern full suspension bike. Bikes are that good these days. I do find my 170mm bike these days collects more dust than the others, but given my past the most success I have achieved on a bike...

I've always been a believer of having some headroom in your bikes capability, so there is at least a little bit of margin for error and forgiveness. As you get faster/more confident its better to "ride in to" your bike rather than reach a point of over riding it. As some riders get more skilled they can choose to run things a bit tighter and trade off a little bit of that forgiveness to get a shorter travel, snappier bike but they understand that there's less room for error. I see it all the time in bike shops that staff...

Pick the damn bike that best suits your trails and your riding. It's not that complicated. Buy the bike that is ideas for 90% of your riding. Don't buy a bike because you will do a one week trip somewhere that needs a bigger/small bike. Rent that bike.