rEvo instructor and also Choptima diver here. Just disclaiming my prejudices.
I would not buy an SF2, period. I know a number of people who have had them, several of them being instructors on the unit. None of them dive them anymore. They were a hot fad for a minute, until everyone realized how terrible they breathe in almost any orientation other than the one orientation where they actually breathe decently. One friend was an SF2 instructor and a cave instructor. When he dived the SF2 in a cave, any time he had to do a head down descent - like going down through a chimney - he would switch to his open circuit bailout, because it just wouldn't breathe in that head down trim. He wouldn't dive the SF2 in the ocean - he dived a Meg instead. His deep wreck diving protocol was to point his scooter at the bottom and hit the trigger to get down. The SF2 wouldn't breathe like that (while the Meg still breathes just fine like that).
If the JJ is the only other option, then I would not have any problem with a JJ. I have not dived one, but I have a number of friends who do or did. At least has upgraded from a JJ to an X CCR. I like the X better, but the JJ would be acceptable.
If you want to travel, and you have Choptima support there, well, even though it's your first unit and I would generally recommend back mount for a first CCR, it would be hard to argue against you going with the Choptima. It is great for travel. Very light weight. Breathes awesome in all orientations.
But, I only use my Choptima for cave diving. I don't like the chest clutter. And I use mine with a sidemount rig and I don't like sidemount for ocean diving.
And I have a rEvo, so I don't NEED to use my Choptima for anything else.
rEvo is my choice for everything except cave diving. It is also great for travel. It does not breathe as well in all orientations as the Choptima. But, it does breathe well in "normal" trim and acceptably well in other trims. And it has many other advantages including being small and lightweight (I used to dive a Mini but moved on to a Micro, which I prefer), the rMS for monitoring scrubber status, the dual scrubbers, the hybrid operation (Constant Mass Flow orifice plus electronic solenoid), it's VERY low profile on your back, gives a VERY clean and uncluttered chest area, and it is "dense" for lack of a better word. It is lightweight and yet also requires little or no lead added for buoyancy. I frequently dive a 5mm wetsuit in the ocean, with 3L steel cylinders on the rEvo and I don't need any lead - not for buoyancy or to get good, horizontal trim.
And it can run up to 5 O2 sensors. You can even fit 6 without too much trouble, but it is actually setup from the factory for 5. People diving with only 3 sensors like to dismiss the value of having 5. Personally, I have found 5 to be very valuable. In short, you can have a controller failure or an O2 sensor board failure on the rEvo. When that happens, you lose use of the 3 sensors connected to that subsytem. With any 3-cell CCR, the O2 sensor board failure would cause you to lose ALL ppO2 monitoring. With the rEvo and 2 additional cells connected to a monitor (like a NERD), you can easily and safely finish your dive staying on the loop as normal.
If the controller and/or O2 board are totally dead and you have more diving to do before you can get replacements, you can move an O2 sensor over from the dead controller/board to the NERD monitor and continue diving the unit, with 3 O2 sensors, operating it as a pure manual unit. You cannot do that with a JJ or other 3-cell unit (that I know of). At least, not if the 3-cells are all connected only to the DiveCAN system in the unit. The rEvo monitor subsystem is COMPLETELY physically independent from the controller subsystem.
And, it is a robust unit. I've bought all of mine used and they just work. My counterlungs are all well past their 5 year service interval and still in very good shape with no need to be replaced.
