You can pay for I-94 using Cash or pay Online on CBP website. The only catch is that, you need to visit within 7 days from the day you do this online and you need carry a print out of the same. If you do not visit within 7 days, there are no refunds. You can pay online for I-94 at CBP DHS.Gov Website  . In terms of which is better, sometimes the line for Payment at Counters can be long...so, it is a better idea to pay online and carry the print out the confirmation.