There isn't a right or wrong answer to this question, but I'm interested in peoples' opinions.
I have a 9700 that is out of warranty and has a sticky keyboard issue. I've replaced the membrane a couple of times but the issue keeps on coming back. It's slowly getting worse... sooner or later the keyboard will, again, be unusable. But... my BlackBerry representative tells me that 2011 will be an exceptionally interesting year for BlackBerry so I don't want to buy a new device with a contract now, but I'd like to wait.
If my 9700's keyboard fails, I'm thinking I'll either buy a new 8520 or a used 9550 (I know somebody with a Storm 2 for sale). Neither is going to be the same as the 9700 for lots of reasons...
The 9550 is tempting as it'll help me support my customers with the handset, whereas the 8520 will drop right in where the 9700 lifts off.