Tbc

I'd like to start playing wow again and i was wondering whether to choose a wotlk realm or a TBC one. I saw that most pl here play on the TBC realms. What are the pros and cons of each? I enjoy both pve and pvp ( If i start at TBC i will roll a mage, in wotlk i will continue playing my 40 pally).

I personally hate northrend, give me outlands all day. Otherwise there really isnt that much of a difference. You get to level up 10 more levels in boring zones.

I would say that WOTLK is more friendly for a player. Leveling is easier imo, there is not that much grinding as in TBC, also WOTLK realms are quite old, that means new player can get even 1k gold from old ones. It's quite easy for now. TBC (outland) is much harder, today I saw warrior who died fighting normal mob on same level (38 war vs 38 panter, both 100% hp, panter won :P). It's very hard to get 60% ridding (40g), while it costs 4-5g on WOTLK. For me, PvE is good, but:
1) Wotlk content is easier. Sometimes you can just go in few mobs, aoe them all. Almost every player killed LK on normal mode, a lot on HC. Also, as I said, server is quite old so if you will find an decent guild they will take you for icc 10-25 with 2k gs and after 3hrs you will be 5k gs :P
2) TBC dungs and raids are harder. Aggro/threat can determinate dps of damage dealers, since they need to slow down or they will "take it". Dungeon's trash need to be CC'ed often.

So, for pve, wotlk is easier. That doesnt mean it's bad, it's really enjoyable and fun! In TBC some ppl will never see part of SWP cause it's not that easy :P
Personally, I prefer TBC now, after years of playing Wotlk, Cata, WoD, Legion I'm just sick of that how easy this game became (lvling - just shamsh that mob in 3 hits, Dungeons - just full aoe dont even look at all, Raids - just run with random ppl by Raid Finder, Death - no more. During lvling you could take even 5+ mobs and thats easy. Here, 2 mobs can be deadly for you and I love it).

Difficulty: TBC>Wotlk
Raids: TBC harder, but both expansion enjoyable. Some ppl say TBC was more "epic".
Gearing: Wotlk for sure easier.
Skills/ridding cost: TBC is more expensive.
Class'es quests: Tbc - almost every class have it (for example poisons quest for rogues), wotlk - I think there are few?

If you are not semi/hard core player, want to play for fun 3 times in a week, 1-3 hours per day I would reccomend Wotlk.
If you want to die quite a lot of times during lvling, make some hard work, even grinding for your gear take TBC.
Also, think about that TBC realm started 20 may. I don't think that there will be any good TBC server like this one in a future, it may be last Chance to play on this expansion in very well-scripted serv :)
If you have more question pm me, we can exchange skype ID and ask me anything, I'm on warmane (molten) almost since beggining ^^

thanks for the reply, will talk to you soon !