Looking at your criteria, I think the most interesting features of a desktop are:

- easy & cheap RAM upgrades

- ability to swap & add/remove harddrives. Running multiple OSes, you can more easily share common data, and you can pull out your main hdd allowing you to experiment safely.

- ability to upgrade the graphics card later if you want to.

If you are constrained by your budget, getting a desktop allows you to upgrade over time more easily. It also allows you to use different and more hardware.

Personally my 1st priority is having a good laptop, but I also have two desktop workstations and I prefer using them over the laptop. But if you use an external monitor, keyboard and mouse you are already getting most of the benefit of a desktop.