The 750tx isn't 'better', it's equally good. The tx750 offers more power, the 620hx offers modular cables. Since 620w is overkill for your rig, and 750w is complete nonsense for your rig, I would go with the 620hx instead, because it's modular you will have less cable clutter in your case, which is good for airflow and simply looks better. Also, megavovans assumption that the 620hx is a multiple rail design is wrong. It's marketed as such, rather long story tbh, but it's in fact a SINGLE 12v rail. So 620hx would be my pick.