Mathura is relegious place as you know but if i've to go for solo trip i'd select jaipur. Its so fascinating rajasthani cuisine and dresses i'd love to try hawelis and colorful streets and so big mansions and especially architecture in jaipur is nothing less than an ancient film set.
Jaipur would be better. I have been to jaipur and mathura both. If you want religious visit then go for mathura and if you want historic visit then go for Jaipur. plan according to your preferred type of visit and also considering budget. Happy planning..!! Happy journey ..!!