Therosterman,
My feeling is that there will eventually be consolidation in the low-fare market in the UK - you have Go, Ryanair, easyJet and Buzz all competing for the same market segment. I would align myself with the eventual leaders - in my opinion Ryanair and easyJet.
I have heard the work conditions at Ryanair are difficult - but short-haul flying is typically difficult with regard to hours/schedules anyways. Both Ryanair and easyJet are loading up on newer-generation airplanes as well - beautiful and advanced 737-800s for Ryanair and nice 737-700s for easyJet. Go has been somewhat undecided about next-generation airplanes (rumors of A319s but nothing substantiated yet).
My advice would be to go with the eventual leaders - either Ryanair or easyJet. Have you seen those cool easyJet banner ads? The ads make it sound like a cool place to work (probably does not fit reality). Anyone disagree?