Thanks for the advice guys...
I intend to use it quite a bit so think V's will be the way to go. One thing I am wondering about though: behind the seatpost on the frame the cable goes round in a metal tube bit (not very clear I know!) - will there be enough clearance after this for the V brake's metal curve bit?
P.S just proof read this and don't think it makes much sense - apologies if noone knows what the hell I'm talking about!
that tube style cable guide means you have an XS (18") frame. there's not enough space to use that tube as housing stop with v-brakes. i'd suggest to use vs on the front and cantis on the rear. can only pose a problem if you use combined brake/shift levers.
the easiest way to run vs on the rear is drilling the cable outlet hole and route the cable housing to the inlet. or drill inlet and outlet and route the housing from the lever to the brakes. take the thinnest cable housing you can find to keep the drilling ot a minimum. i would ALWAYS use cantis instead drilling a frame as i think they work more than well enough on the rear but if you are not that puristic...
a less nice solution would be to route the housing outside of the top tube and zip-tie it to it. or using such an adapter but on the XS frames space is really limited