They are both as reliable as eachother if they are well maintained and not ridden stupidly.
Keep them both topped up with a good 2t oil, such as TTX and run them on optimax and i'd be surprised if one went before the other.
There's so many rumours about 125cc two strokes (especially the RS) blowing up all the time, but that's only if you ping the balls off them in every gear, changing far too late through the revs, or equally if you crunch down hard through the gears and you over rev. Sure, you have to ride them hard to get the most out of them, but a friend of mine has had his RS125 for two years now and taken good care of the bike - has it blown up or caused him any major wallet ache...no.
So now that's out of the question, look at the performance.
The NSR is a skinny bike, skinny tyres, skinny tank and all together flimsy build quality.
A friend of mine had an crash at 30mph, where the bike did nothing but slide along the floor, however the damage was as though it'd been a much worse crash. The fairings obviously scratched - as any bike's would, however they also cracked and bits ripped off. When he inspected the fairing afterwards, he realised just how flimsy they were.
The RS125 is renound for it's build quality, take a look at any that has been dropped in the same was as the NSR above, you'll see road rash to the fairing, but no deep scratches and definately no cracks.
The tyres are nice a wide (sure - they're not superbike wide, but then again it's not a superbike is it
) and the tank is a good size so you can really get your knees tucked in underneath and if you wish, there's plenty of room for your chest on top of the tank to get tucked in behind the screen.
Comparing the performance of the bikes, you will find that the RS comes up trumps in every situation. Derestriced it's faster, 0-60 and 0-100, with a top speed of around 100-110mph standard and then with a BBK + bigger carb you're looking around 120mph - 125mph.
The NSR will top out at around 90mph and will get there much slower.
On corners, the wider tyres on the RS make it much more predictable. You can easily get your knee down without feeling like the bike will scoot out from under you, and in the wet they still provide you with enough confidence to not have to ride everything completely upright.
The NSR tyres are not much wider that the RS50's tyres, 130 on the back I think. The proof is in the pudding as my friend came off on a corner when he lost grip on a roundabout (mentioned above) due to the skinny tyres not finding grip on a damp road - moments after an RS had taken the same roundabout.
So, after much waffle - I conclude that you won't go far wrong with an RS125 - the performance, styling and build quality far exceed that of the NSR.
There are also hundreds of aftermarket bits you can buy for them - just fleabay "RS125" and take a look yourself.
Good luck with the purchase
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. We should just face it both are fun, neither is a clear winner, horses for courses etc!
) and that sounds pretty sweet too 
