Well it seems to be that if your traveling with your camera, then you need to figure out how convienent you need your equiptement to be. If your going to be taking artistic slower pictures, then separate lenses are the way to go, They will probably give you a lilttle better results. However, if you are doing alot of hiking or your taking quick shots when you can get them, the convienence of one lense is really hard to beat. A 28-300 lense isn't going to be the best lense on the market, but i don't think your looking for the best you can get with your other choices.
The biggest selling point i think for you is the convenience of not fiddling with lenses while you are trying to get the shot. With one lense you are less likely to lose it, or the front and back caps that are nessesary to protect the lenses when not in use. (We all have fiddled with a lense and accidently left a lense cap somewhere). For a long trip, if your dragging your camera around the world, i think you should consider the 28-300mm lense. It wiould probably be the best choice for you.
If you are considering another lense though, i would suggest a 50 mm prime lense. For a Cannon i believe they can be had for under a hundred bucks. It doesn't have zoom but it is the lense that most accuratley see's what the human eye see's in terms of distance and vision. This would be the lense that most professionals use the most to make sure what they are seeing, is actually what the camera see's. Plus its a nice fast lense. If you can be good with a 50mm lense, then you can be good with any lense.