For those who are interested, I did some direct benchmarking on the wifi (GT-P5113) and AT&T (SGH-I497) versions. I ran AnTuTu and Quadrant Standard on the tablets in their factory-reset state (after installing the benchmark apps from Google Play, of course). These are the results:
CPU: TI OMAP 4460
Graphics: PowerVR SGX 540
Quadrant: 3,126 (CPU: 4,633; Mem: 2,765; I/O: 6,399; 2D: 251; 3D: 1,580)
CPU: Qualcomm Snapdragon MSM8260A
Graphics: Adreno 225
Quadrant: 5,616 (CPU: 8,877; Mem: 9,583; I/O: 6,425; 2D: 1,014; 3D: 2,180
The AT&T version is the clear winner. That's the one I'll keep, and my brother will get the wifi-only version. It's slower, but at $120 not a bad deal at all.