I have owned and used both cameras, just sold the T3 a few months ago.
I dont think those ridiculously rigorous technical comparisons some websites (like dpreview.com) can provide are of much value to you here, where it seems you simply want to know the real-world differences of the two cameras in general use.
This list might not be exhaustive, but the main things the T3i brings to the table over the T3 are:
- Articulating screen not that big of a deal for those who prefer to look through the viewfinder, as I do. Seems to me, its just one more thing that can break. But these days, people seem to like the screens, and if so, the ability to move it is handy.
- More megapixels this probably isnt going to make the slightest practical difference
- Built-in wireless master flash very nice feature, but useless unless you have fairly pricey Canon speedlights, and know how to use them.
- Aforementioned spot-metering mode. If your wife is like 99% of consumers, she wont ever miss this, assuming she even knows what it is. Besides, she can always use partial metering on the T3, which is similar.
- External microphone input jack for video. If you dont shoot video, who cares? Even if you do, youll probably just use the built-in mic anyway
- Slightly better feel to the camera body. Neither camera is a high end piece, but the T3 does not have any grippy material on the body and as a result has a rather cheap plasticky feel. Probably not a big deal at all, if your wife already likes the camera.
Youre not going to see any differences in image quality between the two cameras in real world use.
Thats really it. I upgraded to the T3i because I wanted the master flash capability and the price was so good that it cost me nothing, after recouping the cash by selling the T3.
If youve got money to burn, go with the T3i I guess, but unless you see $200 worth of value in my list here, Id say go T3.