The PG279Q uses an IPS panel the PG27VQ uses a TN panel. Tn panels typically don't have as accurate of color reproduction as IPS, and worse viewing angles so if you look at a TN panel from an angle colors will be distorted. However TN's tend to have slightly better response times and input lag however that difference is much smaller with these new IPS panels.
IPS panels the PG279Q in particular can have backlight bleed issues, which can be quite unsightly for such an expensive monitor. That is the only reason I would consider the TN over the IPS.
I wouldn't say one is "better" than the other, they are different. I personally would prefer an IPS panel assuming it doesn't have a ton of backlight bleed or IPS glow.