There is no well-defined point where something is LR to my knowledge. Many vendors don't even tell you the resistance.My belief is that they don't tell you the resistance to avoid complaints from people who don't know how to measure resistance properly. But that is another story.
I had been using 1.8 ohms, which is a big difference from the standard 2.4 ohm atty's I'd been using on my standard eGO. Much more vapor, and also shorter life, especially if you're not careful. They seem to have come down from 2.4 ohms to 2.2 ohms as a "standard" resistance. I think this is good, since there is more vapor this way.
I think 2.0 ohms would be on the boundry between standard and LR. I, personally, am planning to order the LR version for my next eGo T order, just because more vapor is nice, and I got about 3 weeks out of my 1.8 ohm LR atty's on the standard eG0 (non
tank.). 2.0 ohms should last even longer, so this is acceptable.
In summary, I'd get the 2.0 ohms, but be aware you have to be more aware of never running the atty dry. When you START to get that slight burnt taste, add more
juice as soon as you can. Actually 2.0 may be a good compromise between more vapor and atty life.
(Since the Riva is basically the same as an eGo everything said above about eGo's should apply.)