Guys, do you have any new revelations to add to this question? I am looking very hard at Alphas now but, I really want to ride them both first because one ride will tell me all I need to know. Before any rides under my belt, I am leaning hard toward a 154" in either a Mountain Cat or Hardcore.
Do you plan on just riding it stock? Here's my .02 cents. I bought a alpha 154 and several of my buddies have 165's. In my opinion, the 154 will do just about everything the 165 will do, unless its really deep. In that scenario, the 165 will definitely out perform and out climb the 154 a bit. However, it most situations, the 154 will do everything and anything you need it to, and it has a certain fun factor with how agile and effortless it is to throw around.
Two things I would also consider, so back to keeping it stock? I put a silber turbo on my 154 and now there are days I wish it was a 165. It's an absolute riot and wheelie machine, but it can be a handful. The other thing is, if you go with an alpha, the 154 is very responsive to rider input and is easy to override. I was frustrated at first, but now that I've adjusted my riding style, its been awesome!
Believe me, either way you will not be upset! You will have a blast on either length and it will be nothing but smiles once you get that thing on the snow. Best of luck!