I think at 648 you can get something better than a secured card... unfortunately, I don't think it's enough to get the regular Quicksilver... you probably need roughly 680 to start with that...
However, I do think you should easily qualify for Quicksilver One ($39 annual fee) and I would think you should be able to get a regular Discover It as well (please correct me if I'm wrong here, I would guess 650ish should be enough)
Both are good cards with pros and cons...
Quicksilver One:
+More chance to qualify
+Mastercard is more widely accepted than Discover
+1.5% cash back is better than 1%
+Capital One is likely a more generous lender, at least initially
-You probably want to keep your first credit card for a long as possible for credit building... annual fee won't help that
Discover It:
+Right now, the cash back is doubled at the end of your first year (but it does go back to 1% after that)
+5% quarterly categories let you earn some decent cash back (although on limited things at a paticular time), plus I hear that the discover deals are nice (earn decent cash back from select merchants)
+No annual fee so you can keep you card forever without having to pay up $40 each year
+You get a real FICO score rather than simulated score from Capital One
-The biggest downside is while Discover is accepted at more and more places now, it's still not as universally accepted as Visa/MC which might be a bit of an issue if you only have one card
Honestly, after hearing your score, I think Discover might actually be better for you IF you are willing to use debit in case your card doesn't get accepted somewhere... No annual fee is a big thing for long term credit building... Which card earns more is debatable based on what you spend your $$ on but overall, I feel like you might not see a significant difference in cash back and it might actually fluctuate from time to time. The only risk with Discover is that you are probably towards the lower end of the application? It says their average approval score is roughly 660, not too off, but lower... I'm pretty sure though that you'll have no issue getting Quicksilver One... it's a difficult decision with no truly easy answer... both will be fine... I see Discover as having more upside while QS1 is more sure thing but you will probably outgrow the card soon...
Must garden until 2/1/2022 to hit my goal AAOA. 
