If you are even remotely concerned with speed, keep in mind that no laptop will compare to an equally powered desktop. For that matter, don't expect a laptop to last as long either.
If you really need the mobility, you obviously need the laptop-otherwise, you are much better off with a desktop.
You can get a decent desktop machine for $600. I also have a laptop, and dread having to use the slow SOB....and it's less than a year old.