Almost all other Contiki tours advise using a suitcase as luggage. Would it be better to use a backpack/duffel bag as primary luggage for this type of tour instead?

no you should be fine with a suticase, just make sure its got wheels!

Found this on the Kumuka travel site..Hope it helps!!

http://www.kumuka.com/Dossier.aspx?TourId=AST7

Luggage and Personal Equipment: How much should I take?

Please bring lightweight walking shoes or sandals, and a day sac. Pack essentially for hot conditions: days are hot and humid. Lightweight rain gear is essential at all times.
Because of limited luggage space on transport, we must limit each passenger to ONE main suitcase or frameless backpack and one day bag. Your main bag must not weigh more than 20 kg’s.
A frameless or internally framed backpack is the best way to carry your belongings since it is easy to lift on and off vehicles.
A small daypack is practical for keeping general items (e.g. camera, books, wallets etc.) at hand when your main pack is out of reach. It is also handy for day trips and excursions. We also recommend that you keep your passport, credit cards travellers checks etc. on you at all times, preferably in a money belt worn under your clothes.
TRAVEL TIP: You might be tempted to buy a few souvenirs here and there, so leave a little room for those extra purchases!!

Would highly recommend a backpack over a wheeled suitcase. The environment ( roads, pavements )in SEA can be quite dirty and dusty in some cities/ towns.There is also not much need to bring lots of clothes as laundry is very affordable as well as new clothes.

I took a suitcase on my europe tours, which were 3 months in total and it was a big pain!
I had to carry it up flights of stairs and drag it over dirt tracks and on and off ferry ramps and by the time i got back to sydney it was destroyed.
Now im doing one month in asia ive gone out and brought a backpack, that also has small wheels. I plan on using it mainly as a packpack, but i have the option of both in a way.

Hope this helps