You'll be happier, and I'll likely do the same thing with my woman's S3, assuming I can pry it form her fingertips. The better graphics and performance, even with the base M3 model, should be noticeable. The keyboard will be night and day.
After I took back my S3 and preordered the SP4 (1125.15 altogether with the keyboard, damn Tennessee sales tax), I spent a good while playing with the demo model. I looked at it all over, compared the thickness to my iPhone 4, carried it a bit. I went nigh on scientific about it.
First, the keyboard. I've already said it was nice, but I'm going to say it again. It's nice. I've always said that the keyboard was the weakest thing about the Surface line, but now...nope, I can't. It's good stuff. Even the trackpad is nice. It has that same smooth, frictionless feel that Mac trackpads have. And just like with the Macs, you barely even have to think about the gestures to perform them. Three fingers up launches Task View, Three fingers left-right swipe through open applications like Alt-Tab (it even brings up the overlay window). Two fingers scroll through pages, about what you'd expect, and a two-finger press is a rightclick.
I don't normally use trackpads unless I absolutely have to, except on Macs. They're too much of a pain in the ass to use anywhere else. To say that the SP4's trackpad is about as effortless and smooth is pretty high praise coming from me. The only downside is that it's a bit more cramped comparatively.
If I have one complaint, it's that the keyboard does have a tiny bit of give when you have it attached to the screen. It's nitpicky, but it could be considered a downside when compared to a traditional laptop keyboard.
Thickness? I played around with the i5 rev, and it's just a smidge thinner than my iPhone 4. It's not as thin as the iPad Air, but really, once you get to a level of thickness where you're comparing differences in millimeters, it becomes more a bragging right than a practical matter.
Weight. Yeah, it has more heft to it than my now old S3 with the type cover on. Though to be fair, it did have a security bar wrapped around the keyboard, so it's hard to make a good judgement call there.
The screen? It's easily as good as the iPads are sporting. That's high praise, because iPad screens are impeccable. On my S3, little icons, like the wifi symbol looked a little pixelated. Here? It's all smooth lines and gentle arches. No complaints. One weird thing I noticed is that the larger screen with the reduced surrounding bezel makes it look weirdly smaller than the SP3 sitting next to it. OPTICAL ILLUSION! :O
The stylus. I only used it in OneNote, but it did seem to be a little more sensitive to pressure. Not a huge difference, but it seems to pick up featherlight touches a little better. The one thing I didn't test was the magnetic dock on the side of the screen. Didn't think about it until I left.
I'd say overall, it's a solid built, well designed machine. I'm glad I'm getting it.