Hey. Depends what the purpose of your twink is.
If you're just looking to exploit your way into battlegrounds and nothing else - go 79. They are strictly stronger in instanced pvp.
If you want to do literally anything else - go 70. They have an actual community of players (albeit small)
I suppose your next questions would be along the lines of "what do you mean by exploiting my way into battlegrounds?" and "what else is there to do?".
First question first: Twinks and levelers have recently been separated into different brackets, meaning twinks no longer get any bg pops whatsoever. However, there is an exploit that allows you to get into the non-twink battlegrounds as a twink. Blizzard have yet to punish anyone for doing this. You can read about how to do it
here.
To answer the latter question: It's going to depend on the region you play in. If you're in north america I can't help you. Maybe
@Blackberryy can tell you about the current state of the 70 community over there. If you're playing on the european realms there are a few regular activities that you can take part in if you're level 70. I can PM you a discord invite if that sounds interesting