Ok, I might getting a little bit anal here but I had both swingarms side by side while trying to stay open-minded and consider both of them for my nsr250. So, I couldn't resist comparing them...
First of, the eternal debate about the weight of both swingarms.
A complete Nsr250 mc28 Proarm measures about 9.800Kgr and an almost complete mc21 swingarm measures about 8.500Kgr.
So, mc21 swingarm is 1.300Kgr lighter but to make things clear, these measurements are really rough.
As you can see from the photo, the mc28 proarm is complete while my mc21 swingarm has quite a few parts missing. Also, I haven't measured mc21 wheel bearings and all 4 metal spacers as these are part of the wheel while on the proarm these are integral parts of the hub-axle assy (hence included on the above measurement).
Overall, I believe that both swingarms weight really close with the mc21 maybe ending up being just a bit lighter.
I really like the robust looks of the gullarm design and tyga-performance sells a kit with all rear spacers made out of aluminum that looks to be a very good upgrade for a little bit more weight saving.
On the other hand, the proarm has a unique design. It's heavier component is the rear axle as it's constructed out of mild steel (about 3.5Kgr just this one alone!). I think that Rs250 nx5 rear axle has really close dimensions and might be a straight swap, with hopefully less weight. If anyone knows for sure, please let me know.
I also like that the proarm has no silent blocks at all and changing the rear tyre is a joy, as you do not have to chase spacers while you remove the wheel and retightening the chain is not necessary.
Overal, I'm a person that likes innovations and open-minded motorcycle design, so the proarm will most probably end up as my choice but both swingarms are really nice and selection between the two is really down to personal taste rather than specs.