I use earnest, sofi gave me garbage rates.
Went with a variable rate that is at 3.5% currently (have over 100k). Their fixed rate they offered me was around 5. Sofi offered me 6% when I was already at 6.25%, garbage.
I would honestly apply to 2 or 3 and just pick the one that gives you the best rate, I really don't think it matters otherwise. Earnest does let you customize your payment/interest rate to basically any payment you are comfortable making. Sofi locks you into specific payments and term limits but that might be the only difference I can think of.
Here is my referral link, but not trying to advertise for one or the other. Pick the best rate you can find.
https://www.earnest.com/invite/timothy378