iPhone Got a iPhone 3GS for free with 4.2.1 on it. Is downgrade possible?

[devilstrider]

I want to unlock it to use along side my Galaxy Nexus but it has baseband 05.15.04. Is it better to just got to 5.1.1 and use it that way on AT&T? I have heard that iOS 5 makes it run slower.

 

[LandonDonovan]

You can save your SHSH with iFaith for 4.2.1 if you don't like 5.1.1, but you can always go down to 4.1. 5.x is actually faster with Safari. I haven't noticed any speed drops, and I much prefer the look and feel of it compared to 4.x. There may be some more idle battery drain(20% or so per day compared to 10%), but that isn't too bad.