My husband and I are currently hip-deep in waiting for his I-485 (Adjustment of Status) application to be completed (which was submitted after an approved I-130 came through...long, complicated story) and I've been researching the US side of spousal immigration for the last 19 months.
Depending on where you live, the Adjustment of Status can take from 6 months to 2 years. That's part of what makes the whole process in the US so frustrating and unfair. People who applied in January are still not approved while people who applied in March and April already have their green cards in hand...just a load of BS, really. But...*shrugs*
It is so true that going from the US to the UK is about a million times easier and more straightforward than the other way around. Yes, you can file the I-130 and I-485 together. Yes, it costs a lot of money to do that...over $550 at the moment, I believe, just for those two, not to mention the additional fees for the EAD and the advance parole paperwork. Those two will probably bring it closer to $800 when all is said and done. And then, on top of that, you'll be paying another $494 for the UK visa...
Now that we've been waiting for so long to get my husband's visa stuff sorted out, we're contemplating moving back to England permanently...and the visions of all the money we 'wasted' on his US immigration just make us shake our head cause there's not much more we can do about it.

If you're going to settle in the UK, don't bother applying for the US immigration. It'll just cause more hassle. It is probably best that he return to the UK before his 3 month period is up, but if you go over by a couple of weeks, I doubt you'll have any problems coming back to visit in the future. From what I understand, they have the most intolerance for those who have overstayed by 180 days...not 15 or 20.
It's always a risk, though, so do what you feel most comfortable with. Applying for the US immigration if you don't intend to use it would just be a waste of time, money, and energy that you could better spend elsewhere.