An MX-based setup does not use the cPanel provider's mail server, so it should be more reliable/secure/convenient in a dispute/etc. If you also use your @gmail.com account to send mail, it has the added benefit of allowing you to migrate off GMail easily, should it ever start to suck (just set up an account elsewhere and change the MX record - people should already have you@yourdomain in their address book).

Additionally, most spam filters take the sending host into account ("this host is known to have sent a million spam messages today - let's not bother talking to it"), but this information is lost (untrustworthy) if you forward via another mail server; it's likely that GMail has a better spam filter than your cPanel provider, so try to give it all the information you can.

There are some minor other issues - faster delivery, less wasted disk space on the cPanel provider's end, message sizes are limited only by GMail - but the above are the main ones.

Do note that you need GMail "for domains" instead of just a @gmail.com address. The "for domains" version is free, though (perhaps up to some upper limit - 50 mailboxes?).