Either? Neither? Which system is more likely to have good rpgs?

Probably both since Final Fantasy, The Elder Scrolls and most RPGs are usually on both systems. It's almost rare these days that one system has exclusives games for just that one unit .

Halo, gears of war, forza..

Those aren't RPG's ----there are some exclusives like those but RPG's....you might find some exclusives on PSN and Xbox Arcade that's not for the other system, but retail exclusive RPG's----its kinda rare.