I have use 3m Wall Graphics, and it's pretty good, but sometimes I get problems with the corners, especially when the wall is cold or a little bit wet.
I have never try the Arlon one, but I heard it was way better than the one from 3m, and a also cheaper.
The main problem is that you are looking for something for about 3 months, so you may not want to use the products I just told you, (unless the pay is good enough), but I think the first thing you need to check out is the texture of the wall. If its flat, with no texture at all, you may use something like a IJ40, or any other calendared vinyl.