You probably need to decide how long you will actually carry it on your back for, and what you plan to do with it. A top loading conventional backpack will probably carry better, have a better harness and padding and feel more comfortable if you are planning any multi day treks like the Inca Trail or the Overland Trail.
On the other hand, they are more difficult to pack and have lots of straps, pockets, buckles and bits to get caught on luggage carousels roof racks etc. If all you plan on doing with it is to carry your kit from bus to hostel to station, you might be better off with a wheeled duffel bag like a Berghaus Mule, that also has a shoulder strap and a rudimentary harness to carry it like a back pack if you feel the need.
As a compromise, there are a number of travel packs around which are a comromise between a backpack and a suitcase, in that they have a zip lid like a suitcase to make packing easy, a reasonable carrying harness that zips away behind a flap for loading on transport, and a fairly clean profile and fewer bits hanging off to get caught on things.
Lowe Alpine do a couple of these, and Macpac probably do the best of them. There is one called the Zambesi (I think) which is fairly bomb proof, but rather pricy. Cheap ones are not worth it, as the large zip is a potential weakness, and I have seen a few of these fail. There are also some monster versions but dont be tempted by anything larger than 65 litres, as you will probably fill it with far too much to make carrying comfortable. Some of them come with a zip off day pack, which is a bonus.
If you fancy any of them, try them on with a load inside, and see how they feel before you buy.
Chris
