If you want to play both DX11 and DX12 exclusive games then you'd definitely want the 550Ti. If raw performance is what you want for games you'd actually be able to play, the 2xx series has a marginal lead. I know a GTX 295 is on par, with a minor lead over a GTX 480.
The only thing is the 285 is gonna run a lot hotter with almost double the 550Ti TDP
Rule of thumb though, always go with the newer architecture~