How long is your rental? For anything over a few days once, it may be more cost-effective to buy your own GPS and buy local maps for wherever you're going (if they're not included in the GPS), than renting a GPS (built-in or portable) from the rental company. It's a lot easier to figure out which GPS's work well when you're buying them than to guess which one will work when you don't know what you'll get from the rental company.

I've never seen built-in navigation in any car that I've ever rented in the US from either Avis or Hertz, in any class between economy and full-size. The only time I've gotten built-in navigation was when I rented a Corolla in Japan a few years ago (from a Toyota Rent-A-Car which was partnered with either Avis or Hertz). That built-in navigation only spoke only Japanese, but by looking at the map whenever it spoke it could tell when I need to make what turns.