The RM2 is a cracking little atty. Very easy to build on. Stock it is a fairly tight draw, it's a mouth to lung atty. I have run mine at any where between 0.5 ohms to 1.5 ohms with good results. Some people open up the air hole for more air than standard, if you decide to do this then take it slow.
Chalice 3 is another small single coil atty. I found it more difficult to build on and it gurgles if you flood it. you need to check for shorts with the cap on before firing as its tight for space. It has the advantage of AFC. I tend to run this at about 1.5 ohms with thinner wire. I like it, having got used to it.
Overall if you can live the fixed air hole the RM2 is easier to build on and cheaper. In terns of vape quality they are very similar.
I have had no leaking issues with either and can pocket the mod no problem. Both are screw tops so the cap will not pop off in your pocket by accident. Neither of these is a cloud chasing atty.
Never tried an RM3 or Origen 16 so I can't comment.
If you have a favourite atty already most can be easily modified for bottom feeding with a dremal or a drill press.