I presently ride a Trek 4900 but I'm considering a the Trek XO1 or the Trek Touring 520 for short to medium tours
I'm not considering ultra long rides (in excess of 300 kms.)
What are the advantages/disadvantages of each bike for this type of riding.
I don't know much about either of them but, on checking out the Trek site, I'd say we're talking apples and oranges here. The XO1 is a relatively aggressive aluminum bike. While the 520 is your classic steel touring bike. I imagine, with cyclocross tires, the 520 would be enjoyable on the trail. If you want something you may want to strip the rack off and hit a cross race (or you just like speed) consider the OX1. If you want something that will dampen your ride with a full load of gear, get the 520.
For long, loaded tours the 520 takes the prize without breathing hard. It has wider gearing, front lowrider braze-ons, bar-end shifters, and a more compliant, comfortable ride. It's a touring bike.
The XO1 is a nice bike and would make an excellent commuter. And you could tour on it if you wanted to, although I'd want lower low gears if I were climbing significant grades with a full load (this applies to a lesser extent to the 520 also).