There's not THAT much difference between 510 and 306 (IMHO)....depending on who you ask, I guess. The threading is an obvious difference, and the drip tips are not interchangeable, but....they both work on the 510 connections. There are fans of each....but either will work fine on your REO....but a RM2 will work better.
The lower the resistance, the warmer the vape. So a 1.5 ohm LR atty of either (510 or 306) will be warmer than a 2.5 ohm atty, either kind. Is that what you're wondering? When you coil your own (rebuildables) you can make the coil hit any ohm you choose, which is one of the beauties of RBAs. Many REO users like a really warm vape and roll coils at around 1.0 ohms. That's about what I like, too.