Those 2 bikes are pretty small for someone your size and holding 50mph may be a problem. May I suggest an Aprilia RS50 instead? It will easily hold 50MPH and will fit a larger rider.
I have owned both the NSR and a YSR. I used the YSR to commute from Ingleside to Sunset using city streets for work for a few months, but that was when I was half the age I am now. I might have hit 50 a few times going down 19th avenue, but it definately would't try holding it there for extended periods.
, SR50 Scoot
, Bangkok Mito, Track Attack Derbi,
Past:'16 & '15 RC390, '11 Madass,'98 Chester, '00 Mito, '12 PW50, KTM BR 65, Aprilia BR50, '06 Aprilia RS125, '04 DRD, 08 WR250X, seven '03 Derbi SM's, 1/2 Dozen 99-04 Aprilia RS50's, two 04+ Derbi GPR's, '00 Aprilia RS250, '99 Aprilia RS250, '95 Cagiva Mito 125, '04 & '95 NSR50R, '90 Honda NS50, '94 Honda RS125, '90 Honda RS80, '88 YSR63 (1st 2stroke!), '89FZ600, '89KLR250(1st Bike!)