Remember the ATT vs VZW iPad (LTE)? If you didn't have LTE in your area, the ATT iPad would bump down to HSPA+ or 3G and the VZW iPad would bump down to 3G.
ATT's 3G is noticeably faster than VZW's 3G...so I dunno. It's a bit of a trade off. If you're with VZW, LTE is going to be fast, but if you don't have LTE in an area, you're back down to comparatively slow 3G. Whereas for ATT's LTE, it may not be as speedy or as prevalent across the country as VZW, but you'll be bumped down to a quicker HSPA+ or 3G speed...so I dunno.
I would personally go ATT though, if it were my choice. LTE/HSPA+/3G (if they follow the iPad) vs LTE/3G that is slower than ATT's 3G (arguably...at least in my area), I would go with ATT.
These are my two cents when evaluating the area in which I live. When I say one carrier is faster than another, I am not speaking for America...I am speaking for this area. Cheers