Seemed like time to re-visit this thread...
I ran the RM3 on my SL Grand for almost 3 months now. I left the air holes stock. - I experimented quite a bit with different
coils... Even though I generally prefer a little cooler vape, I settled in on a pair of 1.1 ohm 29ga kanthal coils (6 wraps on a 2mm shaft). It just really, really produces vapor at .55 ohms.
This morning, I just decided it was rebuild day for my RM2s and my RM3. I've been running an RM2 on my 14500 mini for a couple of months or so, but the 14500 is indeed battery limited, so it has had a ~1.4 ohm-ish coil and even though the flavor is great and it produces OK vapor, it just isn't on an 18650... There's only so much it can do. It isn't a good comparison vs the RM3 on the Grand.
The other RM2 didn't need to be rebuilt, but I wanted to try a smaller diameter coil. I gave it a 1.1 ohm coil wrapped on the same shaft I use for the RM3 coils. And, I ran it all day today. -- Just as I thought before, the airflow is indiscernible -- I can not tell any difference in the draw on the RM2 and RM3. Also, just as I thought before, the flavor is equal. I just can't imagine any atty producing better flavor than these do. And, I just can't say one is better than the other.
There are differences though. The big one, of course, is the increased vapor production and hotter vapor from the dual coil RM3. It makes a denser cloud. This leads to all of those things we all know it leads to such as increased TH and increased
juice consumption. The other big difference is drags per squonk... and that is subjective. In fact, it's almost completely subject to how one chooses to wick their coils. Though, the deck on the RM3 has a bit of a well so it tends to hold on to a little more
juice than the RM2 deck. - Even though it's running duals and I use identicle wicks, the RM3 provides me a few extra drags per squonk. I like that a lot.
The glaring difference is ease of build. The RM2 is dead simple - easy - super-fast. I have aging eyes and my hands just aren't as steady as they were a few years ago. Mounting duals in a small space is tedious for me. I do not like that much at all... What keeps this from being a deal breaker for me is the simple fact it doesn't need to be done very often. In fact, I have yet to rebuild the RM3 because it *needed* to be rebuilt. I did quite a lot of experimenting and built it ~20 times so far, but even as an all day every day 6ml per day atty, this isn't something that will likely *need* to be done even once a month provided I don't go too crazy with the dry burns when cleaning my coils and re-wicking. So, due to that fact, I shouldn't complain so much.
Another noteworthy difference is the caps. I like the RM2 cap because the air hole is where the air hole is. Screw it on, and that's where it goes. All I have to do is be careful when mounting my coil to have the coil exactly where I want the air hole to match up. --> I like the RM3 cap because it's more forgiving as to where the airholes are

If I didn't place one of my coils right exactly where it should be, I can move the air holes a bit and accommodate my carelessness. .... .... .... The other thing about the RM2 cap is that it can tend to drag a dry wick around a little while spinning it on. Sometimes, the right side of a kgd wick just seems to grab it and spin with it. Then I have to put it back where it goes with a toothpick or whatnot before I give it its first squonk. The push on cap on the RM3 does not do that. -- I think I prefer the RM3 cap.
There's no going wrong with either an RM2 or an RM3. There may be details I prefer about this one or that one, but there's just no going wrong with either. - What I foresee for myself: I currently own a 14500 mini, a VVW, and a Grand.
The 14500 will keep an RM2 because it pretty much has to. I don't think there's any acceptable driving of a dual coil no matter the ohms with a 14500.
The VVW will also keep an RM2 because a single coil above 1.2 ohms is the right device for that application... and because that's what I bought my Brass RM2 for to begin with.
The Grand will keep the RM3... and when the Grands start coming with the new SS 510 and I just can't resist buying another grand, that one will very likely also get an RM3 (unless, of course, I also feel a strong desire to satisfy my curiosity about the LP Grands and all of the bigger bore attys that fit them

... never know)
So, Thanks, Papa_Lazarou! I probably wouldn't have tried the RM3 without this thread. I'm glad I did.