The Bag

Whether its a short or long trip, your luggage is your home away from home, the one thing you will bring that will be difficult to replace. If its too big or heavy, you'll curse yourself. If its too small (though many will say this isn't possible), you might regret it. Maybe. No matter the trip, its essential that your bag is easy to tranport and secure.
While preparing for our trip, we were advised by two different friends to get bags that had wheels but also could be used as a backpack.
In ten months of travel, we've used our bags as backpacks fewer than ten times. Though we're thankful to have the option of a backpack, the wheels have been invaluable. If you're travelling to urban centres, through train and bus stations, and not planning to do lengthy treks where you'll need a true backpack, wheels are the only way to go. Its an expensive option perhaps, but well worth it. After all, this will be your home for the time you're away.
I'll briefly mention that a suitcase has very limited use when travelling anywhere but the most developed countries or the fanciest hotels. If you're planning to hire a taxi to take you everywhere, use porters to load and unload your bags, and never need to carry your bag yourself, then perhaps a suitcase is a good option. Otherwise, they're just too cumbersome to travel with for any length of time. Backpacks aren't just for backpackers!
After travelling with it for almost a year, I can wholly recommend Osprey's travel packs. Well designed and solidly built, it has lived through more than its share of rough treatment and shows only cosmetic, minor damage. Worth a look anyway!

http://www.ospreypacks.com/Packs/MeridianSeries/