I've now run a 5 and 3 speed. My opinion is if you can manage the 3 speed, that's what you should probably go with.

1. A bit more efficient (academic)
2. Bulletproof (more or less)
3. A bit lighter
4. Much easier to keep properly adjusted

The last point shouldn't be underestimated. I've notice with my hub that a 1/4 turn will make all the difference between it skipping / missing shifts and running sweetly. (This is my 3rd set of 5 speed internals). I've finally gotten pretty good at feeling and hearing the tell-tale signs that the hub needs adjusting. Now I usually bend over and adjust the chain at longer red lights, but it's still a pain. The real problem with this is that the hubs self-destruct if run out of adjustment for too long.

When I had the 3 speed, I adjusted it once when I first fit the wheel and then again after a few weeks to compensate for the new cable switching. that's it.

I live in Glasgow, where it's not exactly flat, & I managed fine with the 3 speed. I'm not the strongest rider in the world either. The only reason I need to stick it out with 5 speed is that I pull my two kids in a trailer pretty regularly with the bike so need the extra range. (I've also got a double up front, but that is a different topic).

So, the 3 speed is really very trouble free, and the 5 speed needs a lot more attention to keep it happy.

If you do go with the 5 speed, I highly recommend using the metal shifter. The all plastic one is horrible.

Or, you could go whole-hog and fit the S-RF5(N) with the 2 speed rear mech, and have a 10 speed Brompton...