Hey man. 
In my experience, players at $7 play slightly better than those at $3.5.
Mostly weak tight passive players at both levels though.
At a guess, the ratio of aspiring regs to weak tight fish to spewtard fish at each level might be something like:
@ $3.5 level: aspiring regs = 10%, weak tight fish = 80%, spewtard fish = 10%
@ $7 level: aspiring regs = 10%, weak tight fish = 85%, spewtard fish = 5%
 
^^ I've played about 1300 games at each level and this is just based my impression of these populations (take it for what it's worth)
I rarely get rematches though (fish tend to just play you once then swim off, especially the weak tight players who don't like being pushed around).
 
If you're just in it for the money neither level will yield a great hourly rate. 
Roughly speaking a winning player can make about 3% ROI and play 15 games per hour per table. 
This is about 0.5 buyins per hour per table. 
But most of your edge comes from adjusting against your particular opponent so multitabling will likely eat into your ROI quite hard if you're not a good multitasker.
 
@ $3.5 level ~ $1.75 / h / t
@ $7 level ~ $3.5 / h / t
 
If you're learning the game, I think it's best to single table to really focus on each decision. 
The swings can be quite ferrocious so if you're prone to tilting you should stay at the lower level until you're confident you can beat the game (approx >= 52% win rate). 
Playing with a negatve edge + tilt + mass volume in heads up hyper turbos can leave your bankroll on life support very quickly.
Being in a hurry to go from making $1.75 / h to $3.5 / h at the expense of your game not developing seems extremely -EV long term when your game won't be capable of beating any higher games where you might be able to make something resembling a decent return on your time.