i play both of them, used to play 70s very very often. this is my opinion
19 pros:
- games are typically more competitive than 70 (pug bgs)
- it takes a very short amount of time to level to 19
- classes are easy to learn due to having very few abilities (lower skill cap)
- players are generally more objective-minded in bgs
19 cons:
- no arena, only way to do it is wargames
- games can become very boring if teams are continually one-sided
- almost every class has some sort of BiS gear that is grandfathered or costs tons of gold/money
- games only pop during prime time
- professions can be annoying if you have no friend willing to help out
- at 19 when you leave a bg, you need to queue fast in order to get the next game. if you have a slow computer, you will almost always miss the next game and have to wait 20 minutes every queue
70 pros:
- arena happens, from my experience queueing lately there is almost always a team queueing
- more advanced gameplay (more abilities = higher skill cap)
- more battlegrounds are available to play
- rated battlegrounds happen occasionally (although you will have to know people organizing them)
70 cons:
- battlegrounds usually take a very long time to pop (20 - 45 min)
- multiple people with engineering tinkers can ruin competitive pvp by abusing them
- 70 characters can take weeks/months to completely gear out because of raid locks on sunwell / black temple
- leveling professions takes either a lot of farming or a lot of gold
so idk, both brackets have something to offer. the past week or so i've been queueing arena at 70 from around 10am - 2 pm, then hopping on my 19 and playing that with friends when 19 warsongs start popping.