I have recently transitioned across to the Diners+MC combo and the advantages for me are:
- even though Diners is accepted by less merchants, if you're after Virgin points, with 1.25 on the MC and 3 on the Diners, depending on your merchant mix you'll probably earn more per dollar on average than on the Amex Velocity Platinum, for a similar fee ($389 vs $349 for the Amex). Plus you can warehouse the points with Diners and wait for transfer bonuses which Virgin periodically offers.
- I like Starwood Preferred Guest points, this gives you the best earn to that program in Australia (0.625 for the MC and 1.5 for the Diners) - which if transferring in 20,000 lots to most of their airline partners (
https://www.starwoodhotels.com/preferredguest/account/starpoints/transfer/airline_partner_list.html), amounts to 0.78125 for the MC and 1.875 for the Diners in most of those programs
- although the lounge program doesn't include any in Australia or New Zealand, the lounges elsewhere cover a fairly comprehensive list of airports and terminals so I don't need to chase status to get lounge access when travelling overseas
- the travel insurance included is pretty comprehensive and (I think but not sure?) is now better than the Amex Platinum after the recent changes Amex made
- it's not a "forced earn" Qantas card!!!