I use both. Depends on the opponent, and what they know how to use or are willing to learn how to use. I am new to wargaming so I learned how to use both. I prefer TTS because I want to recreate the feel of sitting opposite someone at a table playing a game. TTS has more manipulating involved that can prolong I game, but I don't mind. VASSAL does have some scripts that are useful during game play. The most annoying thing about VASSAL to me though is limited camera angles with respect to map. Ultimately, you should try both in my opinion. A lot of it boils down to personal preference. They both allow you to play someone virtually, but they deliver different experiences. What kind of experience you value more is up to you.
Regarding finding opponents, I posted in this forum, but did not find any opponents this way. I found my opponents either via Facebook, BGG or Discord.