I had to go back to work when my son was 15 mos because my husband was laid off. I hated it. I was so thankful and happy when he got hired back after 6 mos and I could stay home again. I actually was able to continue working a shift or two a week as I wanted after I technically "quit" but fizzled out on that because I didn't want to do it. Somedays now after having worked I miss the interaction with people, but I get that with play dates and stuff too.
I didn't really have to do a transition, since hubby was laid off he stayed home. I can only imagine that it would have been even harder for me if I'd been putting him in daycare.
I DO think however, that a few days a week of preschool are good for everyone concerned. Kids (especially boys) need that social training to get ready for school, in my opinion. And my son is very smart, but I don't feel like I'd be the best choice to challenge and teach him (not that I think preschool is for challenging academics, but fun learning is important too.) If you can work some while he will be at preschool and be happier and more satisfied for it, it sounds like a good thing. Personally, I plan to do things like housework while mine is at preschool, since I never get them done while he is home and awake. But a few weeks of that and I might be looking for a job just to get a maid- lol!
You are only torn because you are a mama who loves her baby! That is a good thing. If you really look at it as the best of both worlds, that sounds like your answer. And just remember, it is not set in stone. You are allowed to change your mind. If it doesn't work out, you can still go back to staying at home. If you go back to work, I would recommend giving it a couple months, that is how long the adjustment to working took me. And after that, see if you really feel like you are getting the best of both worlds.
Good luck in your decision! Regardless, your son will still be well loved and happy. :)