Performance wise:
Both of them are equal since both of them got ETA A and LAMBDA A+ certifications from Cybernetics.
Corsair, report:
https://www.cybenetics.com/d/cybenetics_GGT.pdf
Seasonic, report:
https://www.cybenetics.com/d/cybenetics_SAQ.pdf
Size wise:
Corsair unit has dimensions of: 150mm x 86mm x 180mm
While Seasonic unit has dimensions of: 150mm x 86mm x 140mm
While 4cm may not seem much of space, in small cases, it can be deal breaker when in comes to the cable management.
Age wise:
Corsair HX750i came out in 2014.
Seasonic Focus+ 750 80+ Platinum came out in 2018.
Making Seasonic unit much newer tech in PSU world.
OEM:
Corsair HX750i is made by Channel Well Technology.
Seasonic Focus+ 750 80+ Platinum is made by Seasonic itself.
I prefer to buy my PSUs directly from OEM (e.g Seasonic) and leave the middleman (e.g Corsair) out of the loop but that's just me.
Warranty:
Both units come with 10 years of warranty.
HX750i, specs:
https://www.corsair.com/us/en/Power/Plug-Type/hxi-series-config/p/CP-9020072-NA
Focus+ 750 80+ Platinum, specs:
https://seasonic.com/focus-plus-platinum
Overall:
I'd go with Seasonic because it has smaller dimensions while performance is equal to HX750i.