[Swar]

I don't have experience with the street grip setup, I'm sure it's a great improvement over stock, but I do know bags... night and day difference for me, ride quality and handling.
But bags are a lot more involved. The right psi is key. And don't expect a lower stance by just replacing stock springs with bags, it may even ride higher depending on what psi you want for ride height.
And you'll actually gain hauling capabilities with bags. I have Air Lift bags in the rear rated at 2500 lbs each. You can't get that with a softer spring.
If you're building a drift machine or something strictly for racing on a track, then you might stick to springs, otherwise either setup is a huge improvement for overall driving.
And that concludes everything I've learned so far!