A wire takes place in real time, whereas an ACH goes into a batch which is cleared later, like a paper check.

If you want the funds transferred instantly and with certainty, use a wire and pay the fee for the real-time service.

If not, use an ACH (cheap or free). But be aware the funds won't show up for a day or two. And possibly never, if, for example, the sender's account has insufficient funds or doesn't exist.