Factory Unlocked 4.2.1: Pros & Cons of Updating to 4.x or 5.x esp when Jailbroken?

I the iPhone 4 from the UK and got it Factory unlocked via the IMEI/iTunes official Unlock method.

Its currently on 4.2.1 and I am wondering if I should update it before migrating everything on to it?

I want all the Cydia apps etc.. Especially SpringBoard, Full Bluetooth etc to work well on it.

What benefits / pros / cons would this entail?

Thoughts?

@ Oletheros - What would be a good OS to move to now? 5.0.1 or 5.1.0 or a certain 4.x OS?