Neither are that great, however Kias are particularly awful. The Sentra could be OK since it has a manual transmission. (The worst part of Nissan's cars is the miserable failure-prone Jatco CVT automatic.) However be sure to have a good mechanic check it out thoroughly before purchase.

Echoing the previous response, get a thorough inspection by a mechanic done before buying either one.

That said, if it checks out,  I'd lean towards the Sentra, or at least well away from the Rio.  

I rented a Rio once.   I found it very uncomfortable.  The seat padding is thin, and the seat is not designed for bigger folks like me.  I literally had to fold up a blanket and put it on the seat to drive more than a short hop, because the seat frame was digging into my backside.

Also, Hyundai's and Kia's tend to wear out after 10 years/100,000 miles.   I've owned two Elantras (2002, 2010) and got solid service out of them.  But towards the end, they started falling apart. 

Also, keep in mind the 2009 Rio uses a timing belt that needs to be changed ever 60,000 miles, rather than a timing chain like the 2016 Sentra.