Well, it depends on whether you really need portability. If you don't, then get the tower. It'll most likely outlast any laptop.
If you already have a desktop it's good to get a subnotebook.
If you don't have a desktop then it's best to get the best laptop out there, as it will have to take the place of a desktop.
If have no real need for a laptop, then it's best to get a desktop as you can always add stuff to it and keep it up-to-date.