Either one will work, as will any DSLR produced in the past — I am not making this up — decade. 8x10 is not very demanding. In terms of pure pixel resolution from the sensor, you only need about 1600x2000 for a decent 8x10 — and that's 3 megapixels.

The Canon T3 and T3i (a.k.a. 1100D and 600D) are both way beyond the technical requirements for that, such that everything else being equal you probably can't tell the difference at 8x10 even when looking closely. At the slightly larger 10x13, the higher resolution of the T3i may be apparent, but it's going to be secondary to more important factors. These are:

• Technique: use a tripod, or high enough shutter speeds to freeze camera shake. Make sure you nail focus, especially if using a wider aperture.
• Lighting: noise at higher ISOs will make resolution irrelevant, and the more expensive T3i is just negligably better in this area (practically speaking, the noise performance is the same). Plus, for portraits overall, good lighting is more important than the absolute level of detail you've captured.
• Lens: the basic kit lens that's bundled with these cameras is a good deal for the price, but isn't anything amazing. To really push the limits of either camera, you'll want something better. But, it's still perfectly technically fine for 8x10s. (It's those sports photos where it will really disappoint.)