Personally i would stay in the USA get your hours built up at your employers expense and try to do some variation in your flying if possible, (not all C150 hours or whatever) then come back to the UK and convert it all again, "free hours" are hard to come by in the Uk i think if you are looking for a job, but if you have the offer of instructing and it sounds like you do, then i would stay there, till the end of your visa!
Just my thoughts anyway, im sure other people will disagree and maybe someone might agree but i doubt it!