If you put in significant time with either resource it will yield the results you're looking for so I wouldn't agonize over the decision too much. I used FC heavily for about 9 months at the beginning of M1 then switched to Zanki this past summer. I think zanki is superior by a decent margin. There a many different factors to consider but these are some of the highlights:
1. the anki algorithm is better and it's much more transparent. You're never wondering whether you know a card or when / if you'll ever see it again.
2. images. you have to click like 5 times just to see some crappy picture related to the card on FC. In zanki the picture from UW, sketchy, etc. is right there for you to review. This is really important if you're planning to use sketchy for micro and pharm.
3. customizability. with anki you can edit cards, add in cards, pull out a select group of cards that give you trouble. It's much more versatile than FC.
4. The card style. I think this is debatable because the FC probably form a stronger connection by requiring more detailed / involved answers but I personally don't think flashcards are the optimal medium for that. When a card is asking 5 things and you know 3/5 how do you rate it in FC? That was never really clear to me. With zanki it's 1 or 2 facts and you either know it or you don't. Cram all the facts in with zanki and test your knowledge in qbanks.
this is an older thread of people discussing anki vs FC that you may also find useful
Firecracker vs brosencephalon