I prefer the SR Cisco 306's for more subtle flavors and the 510-t for everything else. The regular 510 doesn't work nearly as well for me as the 510-t.
If you get the 306's from Avid Vapor, I would suggest getting the drip-tip adapter from them too. That way you can use the same type of drip-tip as you do for your 510's. Plus, that way you don't have to drip
through the drip-tip and maybe get
e-liquid on your lips. It can be very irritating.
I like the SR 306's because the LR are way too hot for me. But, you might try both. Most people seem to prefer the LR. And, I prefer the 510-t over the 510 because it seems to wick better without flooding as easily.