The concept of the frost-free is that since the guts are located inside the house (and hopefully heated!), there's no water in the outer portion of the valve to freeze, so if you occasionally use the outside faucet or forget to turn it off for an early cold spell, you won't end up with a burst pipe. Keep in mind that (with one exception), if you leave the hose on it, the water can't drain, and it will still happen.
Most places require an anti-siphon (backflow preventer) to be part of an outside faucet. This is to prevent sucking in contaminants if there's a hiccup in the supply system while the open hose is sitting in say a bucket or a puddle. There are inexpensive add-on versions if the valve you buy doesn't have one built-in.
So, depends on what you want.