A couple of thoughts

Laptop
- Assume laptops are not upgradable
- heat generated by a dedicated GPU on a laptop is significant
- laptops are portable yes, but most people game in the same location
- many people overestimate just how much they'll be taking their laptop outside of the home
- modern laptops are moving towards more limited peripheral support (only one usb, no headphone jack, etc)

Tower
- tower is always upgradable (to an extent)
- heat isn't as much of an issue
- performance is almost always better per dollar spent
- takes up more space