I played both expansions for a considerable amount of time. Things were different then, however.
TBC I mained a BM hunter and warrior. I was on a small server and transferred to a larger server mid-way through it. I did many BGs and would grind until my entire set was full of PvP epics. It took a long time, but PvP was fun, and I remember it being more based on skill rather than gear (although gear helped).
On the small server my guild could barely clear Kara. Many players still clicked spells and key turned. On the large server many people were pugging kara and even BT. But it was unlikely that the pug would clear BT.
Arena in TBC seemed to be a lot of druids and warlocks. Druids were nearly impossible to kill. Warlocks were killable but very annoying.
AV was a lot of fun in TBC. Though it was not often to be Zergs, and alliance won 90% of the time. It gave tons of honor during AV weekend.
Obtaining honor took a lot of time, and you needed upwards of 45,550 or something for a chest or helm. It could have even been 65,550 for all I can remember. It would take you hours on end to grind that much.
WOTLK occurs and I mained a elemental/enhance shaman and rogue. I got 2200 in area as a rogue and was one of the best on the server (even named in a forum post titled, “the best of our PvPers.” I lost the link to it sadly.
My shaman really owned as elemental, and that was the best I remember the state of the game and my enjoyment of it. It was competitive, and I was progressing in heroic ToC 25 man at the time. I left the raid scene and continued PvP. I did not like the PvP sets in WOTLK as much because they did not look as good initially.
Though I think WOTLK offered great content and had some incredible zones at the time. Ulduar was also incredible. WOTLK began the first of the GDKP runs, and I made lots of money participating in them.
Looking back at both expansions, I see TBC as an overall fun and engaging expansion, but I never excelled during it. In WOTLK I got good at PvP and PvE, but it began to go downhill after ICC. Cataclysm was unique in that it changed the original zones, but the state of the game began its first leap into homogenization, a problem that had to be addressed in WoD.
TLDR: WOTLK was the better of the two expansions, but I enjoyed BC more and for longer given that I started it from level 1.