I played retail till "Beta for Azerite" and a bit on a few cata private Servers... Now i am here, i saw that the cata realm closed and now i am looking for an enjoyable experience and wanted to know what are the advandages and disadvantages between wotlk and TBC and what realm/faction would be good to play on....
There aren't clear advantages/disadvantages because preference is subjective. I would say that the difference between the two is that TBC is older, meaning there are less automatic, built-in features. As a result, players are expected to do more manually. As an example, no automatic dungeon finder means you have to interact with players to assemble a group.
Both expansions have a fantastic and rich story.
Play what you enjoy. If you're unsure, try a few out on a server with higher rates. The trouble with someone else suggesting what you should play may end up with you hating the play-style but feeling you've already invested X amount of time and have to stick it out. Play a few classes on one of the higher levelling rate servers and get a feel for what you enjoy.
For server statistics (if that matters to you), see the 'INFORMATION' tab on the website at the top of the screen.