In terms of reputation, they are similiar.
RGU is well known for it's business faculty and links with the oil and gas industry.
Stirling is well known for it's sport's science and life-sciences.
Check out the university tables to see which is better regarded for your subject?
I'd personally prefer to be based in Aberdeen rather than Stirling, and I'm sure the nightlife is better there. Also, RGU has the brand new campus - may be exciting.
Hope that helped!