For me, it'd be a no-brainer: Keep the SP3 because I don't care for the Mac OS interface paradigm and I'm a .NET developer.
That said, you may have different criteria/needs. I think that's what some people are getting at, when they're asking what you'll use it for. If you do mostly web browsing, work with documents, listen to music, and watch movies, then a Mac will do what you need. It, obviously, will have great interaction with iTunes. MS Office is available for Mac. Even things like Quicken are available for Mac. You won't find as many games, but a couple of quick searches show that Diablo 3 is available and Steam has a pretty good heap -- if you were a hardcore gamer, then you'd want to build your own rig, anyway.
I don't think it actually matters to 80% or more of home users. We're really at the point where many, if not most, folks could get away with only an iPad and (maybe) a keyboard.
My suggestion is to wander into Best Buy and try out a Mac (the model you'd buy, so you get the full feel) and play with your SP3 some. See which one you like better, in terms of general flow of the UX. Then, think about your use cases: If you're using at a desk or table, the hinge vs. kickstand is less of an issue because it'll be supported either way. If you're spending time on the couch, will it be writing a lot of text (MS Word, big emails, long forum posts, etc.) or mostly consuming (browsing the web, short emails, short posts, etc.)? The former makes the Mac more attractive, while the latter would be well served by something that could be held as a tablet (it pretty well describes how one of my friend uses his iPad). You'll also have to decide how big of a factor handwriting is for you (it's a big deal, for me).
Both have similar specs, so you won't see much difference in day-to-day performance, portability, etc. It's all very subjective. You might want to ask yourself how much effort you're willing to put into the decision, though. If you're willing to do a lot, then it might just be that you're looking for an excuse to jump ship, which is fine. If not, then you could just go with the machine that doesn't require you to do much to keep. If the event of a coin flip, I'd go with lowest effort.