Having owned both a mk1 Focus RS and a cossie, I would say go for the Focus.

Cossie's can be fun and go well when set up right etc but getting bits for them now is getting increasingly difficult and that can get very frustrating at times. Also not easy to find a good one for reasonable money nowadays

The focus I had I really enjoyed. It wasn't huge power but it handled really well and was really reliable. I think they're a great looking car as well. I'd have another in a heartbeat.