How do contract phones compare to prepaid in terms of cost?
You probably can't get a contract phone anyway. Contracts with a phone included are two years long, but as your residency permit will not be valid for two years, you can not sign a two-year contract. Although I suppose you could probably sign a one-year contract which doesn't give you a 'free' phone.
I'm with Era and find the customer service to be non-existent (I've been complaining about one fault for ten months and they still haven't fixed it) and I have no 3G signal in my apartment (which is right in the centre of Warsaw). My girlfriend and her elder daughter are both on Play and don't seem to have any problems with signal reception.