Either will do you fine. I'd narrowed my 10mm hunt down to the 510 or 20SF, and went with the 510. I liked all the features it comes with, like my FN FNP .45 Tac, and thought it was neat to have something new.
If money is an issue, then are you planning on shooting with a suppressor, as if you are, the 20SF will need at least a threaded barrel at 150 bucks or so, plus if you want them, suppressor height sights (personally I can't hit worth a damn with with "shoot through the can" method). If so, then it's another 100-150 for those.
Now it doesn't look from the pictures of the 20SF that they are milled for a red dot, so that'll be more - I haven't had milling done but what is that - 100 bucks or so?
So about 400 bucks to get a Glock 20SF with the same features as the 510. GB prices seem to be about $550 for a 20SF, compared to $950-1000 for a 510. So price is a wash for getting a 20SF to 510 features - it you want those.
One advantage a 20SF has it that it's a Gen 3, so you can find whatever parts you want, and they'll never go out of availiblity, while FN probably has a 10mm in design that will use about 5% of the same parts as the 510 - it's the way FN operates.