Chak the 3.1 has an o-ring between the top cap and the tank , I figure it's a little more wear on that o-ring filling from the top , I took one look at that sideport on the FT 3.1 and figured that the valve spring would have to be tiny and that spells eventual failure to me no matter what you make it out of , always a chance of accidentally shoving debris in there as well if you shove your mod in a pocket like I do . Don't know why flipping the atty upside down should be an issue for Dusty , ya gotta flip the KFL upside down to fill or go through the top either way ya gotta flip it . The 3.1 has more airflow than the R91% clone and I'm able to get more flavor out of it I would say it's just me but BK came to the same conclusion and I've read that some people are removing the AFC screw completely from the KFL + , the R91% clone has other issues that I already mentioned in a previous post so I'm sticking with the 3.1's , if hcigar makes a KFL clone I'll definitely take a good look at it , I'm really impressed with this 3.1