Hi Mike,
While I'm not Stu I may be able to provide my feedback after trying out the RS5 and RS7 in a store here in NSW (I'll get the OK from Ricktas first before stating who they are, since Stu is a lovely supporter of AP
).
1) I didn't try putting an iphone in the pockets, but after looking through the zips and pouches there is a fair bit of room available on this strap. The strap is significantly fatter than the RS7 (this may be a factor) and is straight, instead of curved like the RS7 (yet another comfort factor). However, a nice pro is the pocket strap is magnetically tied down, so no velcro ripping noises.
2) More a comfort thing as well here, would be hard to explain. I personally found the RS7 seems to sit better on my shoulders, than the RS5.
3) Difference between the FastenR-2 and FastenR-3 is more the footprint of the attachment, so that would be a personal difference too. The 3 is a D-ring, only take up the exact size of it's base footprint with no overhang and probably even more important if shooting portrait (due to the ring on the R2 slopping down).
Side note, I also have 2 Joey-2 at the moment on my RS7, fits an iphone pretty snug too. My reason for the choice was my needs at the moment are more lightweight/adaptable for the varying situation. I'm almost even rempted to remove the velcro on these and put in my own magnetic locking of my own for the silence (which can be a huge boon that the RS5 has over the RS7 right now.
I'm sure Stu could provide even more feedback for you