Realistically I (and everyone else here) have no idea what Apple is going to do in the future. They could phase out the 17" or they could eventually adapt the new style, no one will know for sure until rumors start rolling in.
With that being said it will have to come down to personal preference because both the "new" 15" MBP and the "old" 17" run FCS2 just fine. I currently run final cut on my 15" MBP but i would definably prefer a 17" (which will be my next notebook, when they release a quad core version) and some people want to trade in their 17" MBP's for the 15". It's whatever you want out of your notebook, portability / screen real estate. Although the "new" 15" MBP WILL run it faster because of a faster FSB and faster clock speed processors, which pretty much everything in final cut can benefit from. Motion will benefit as well because of the faster GPU.
I really don't want to judge the new MBP now because i haven't seen it in person, so i can't say if the glossy screen is going to be a problem. I'm not biest towards matte either because my MBP has a glossy screen on it and i have no trouble with it at all, but a piece of glass between me and the LCD.... I'd have to see it in person to make a decision if i like it or not.
If you want / need it now, get the new 15". If you can wait, then wait to see what's in store for the 17" down the road.