My download speeds vary so very much (socks5 proxy).

I understand TCP Only vs UDP Only.

But how can the effective download speeds be so different (apparently) for TCP > UDP versus UDP > TCP? And why is there also TCP = UDP option?

So many questions! Please advise? :)

Thanks!

This is not a preference for speed. It's an option that controls whether TCP or UDP should be used first to talk to peers that support both (many, or most, clients). Unless you have a valid reason, and a well understood problem to solve, you should not mess with those.

Does your unstable (read: free hacked) proxy support UDP tunneling at all? Are you sure?