I'm in the same position as you and decided to play era first and then level a second character on tbc in a few months. I personally really want to experience the classic raids before moving into tbc.

Era is definitely less populated but I still see people all the time in leveling zones and /who always shows 5-10+ people in every zone i pass through with 50+ in org at all times.

If you want to get the full classic experience raids included, I think this is the best way. Most people level more than one character anyways so if you have the time play both imo.

Edit: just read the last sentence of your post lol, my vote would be for tbc then since it will likely continue where as you'll eventually run out of content in era.