The spectrum is about 6MHz wide across the country, and 12MHz in NY, Boston, Philly, SF, and LA, in the 700Mhz band. Because of the narrow channel, this spectrum is most likely to deploy LTE (4G) networks outside these areas. It would possible to deploy 3G networks on this spectrum in those four areas, but I would guess not because 4G is more spectrum efficient than 3G.
So, unless you have an LTE phone, it wouldn't improve your coverage.
Verizon has been very aggressive in buying larger, contiguous chunks of spectrum (>10Mhz wide) in the last decade, even if they had to pay more money to get them. T-mobile got some (that's why ATT wanted to buy them), but AT&T often sat out (or was out bid). Based on just that, I would guess Verizon's coverage would be better for the next decade.
In fact, I have noticed AT&T has a history of under investing in their infrastructure in the last decade. Instead of planning ahead, they defer infrastructure upgrades until the last minute, which costs more but get less return (or no return, in the case of T-mobile acquisition). YMMV.