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?

not a free hacked proxy... why would you assume that?

If anyone could explain the TCP > UDP options and its variations and why the defaults are as they are... Do share please :) Thank you!

I'm not a total expert but here's my take:

The TCP protocol itself (TCP, not what Tixati might be doing on top of it) has checks in place to detect packet loss and corruption. If you're using an internet connection that has a tolerable packet loss, you might have better performance by sticking to TCP, as it would allow quicker detection and handling of these errors, speeding up your downloads and uploads. If your connection is perfectly fine, then you have no need for the excessive checks of TCP (but the peers you connect to might need them), and if your connection is too lossy, then the TCP checks might clog the line and interfere with actual file transfer work.

The BitTorrent over UDP protocol actually uses a transport protocol between the BitTorrent protocol and UDP, called uTP (uTorrent Transport Protocol). uTP has a special way of detecting and handling when your internet connection is busy with other traffic or users (this is called "congestion control"): uTP's congestion control was designed to not interfere with other traffic it coexists with, by eagerly reducing its own consumption of network bandwidth. If using BitTorrent in your network causes lag spikes for other activities or people, you might want to stick to UDP, but this might reduce your BitTorrent speed.

So I'd say:
1. If your network is perfect: prefer UDP over TCP. But if you get slow BitTorrent speeds, preferring TCP over UDP might help by avoiding uTP's peaceful congestion control algorithm.
2. If your network has some/occasional packet loss (e.g. Wi-Fi with good reception): prefer TCP over UDP. But if your network has lag spikes from your BitTorrent, then prefer UDP over TCP.
3. If your network has major packet loss: prefer UDP over TCP.

Maybe other people have other opinions about it.

The '<' and '>' options let you choose which you prefer better, and '=' for equally preferred.