I just joined the R-Strap Brigade and bought an RS-4, RS-5 and CoupleR. The RS-4 has a pouch for memory cards and the RS-5 has two separate pouches for memory cards and business cards and a phone pouch that is big enough for an iPhone or Blackberry. I'm not sure if the RS-5 pouches are deep enough for UK size business cards (I haven't tried to fit mine in yet), but I'll check that out later. I also bought the CoupleR pack, which contains two studded straps that join the RS-4 & 5 together to make a holster style double strap.
I thought long and hard about getting this set-up or an RS DR-1 double strap, which can be split to use one side as a single R-Strap. There's not a lot of difference in the cost, so I ended up going for the set-up that I thought would be the most versatile.
The R-Straps are fantastic to use and very comfortable. I often have two Nikon D300(s) with a 17-55mm on one and a 70-200mm on the other plus battery grips. I can have these hanging on my shoulders for up to eight hours and it really gives me a sore back. I haven't tested the R-Straps over a long shoot yet, but I'm pretty sure they'll be a huge improvement on the standard Nikon straps. I highly recommend getting an R-Strap. If, like me, you often use a two camera set-up, I really couldn't say what the best choice is. I don't think it makes a big difference if you go for the double strap RS DR-1 or like me, the two R-Straps and the CoupleR to join them up. I don't think you would be disappointed either way.