I am thinking of which PSU I should get for my rig, but I have come to narrow it down to two options. A Seasonic 620 watt modular PSU, or a Semi-Modular Corsair CX750M.
seasonic is the best psu brand but id get the corsair for the extra wattage
Definitely Corsair. But I have one so i'm biased.
You're talking about the SeaSonic S12II 620 Bronze? I would choose that over the CX750M. A fairer matchup is the Corsair TX650 V2, which is currently six dollars more than the CX750M. I would choose the Corsair in that situation.
Losing the semi-modular feature by going to the TX is unfortunate, but you already considered it with the Seasonic.
They're both good PSUs. Just get whichever one you find the best deal on.