How to decide between them
The deciding question is whether you can judge engineering quality yourself. Toptal exists to do that judging for you — its screening and match process is the product, and the margin on every hour pays for it. If you have no way to read a portfolio, check live shipped work, or run a technical reference call, that service has real value. If you can do those things, you are paying for vetting you could perform in an afternoon.
A second factor is accountability shape. Toptal owns the relationship and offers a replacement if a match fails, which suits procurement teams that need a named vendor and a contract escape hatch. Going direct means you own the vendor risk but also own the relationship — the person you interview is the person who writes the code, with no account manager in between.