Overall, I prefer a 15" with my A1200 and sd/ff as an allround monitor. 17" is ok for the highest workbenchresolutions, but not so nice in the case of gameplaying where games use 320x256 resolutions.
So, if you're playing games ánd using workbenchstuff, go for 15". If your primary goal is Workbench thingies, a 17" will also do.
