I'd prefer a laptop. If you do only big recording gigs, it doesn't matter. But if you just go track a few tracks, for ex some sax lines or something, you don't need all that. Just take the laptop, and a small rack with the external soundcard and some pres. One micstand, a mic and cables and you're set...
I'm also gigging with my laptop. A rackmounted PC would be workable, but you can't just put that on top of your keyboard rig... And, having a pc rackmounted in a decent flightcase that would allow you to take it on the road, would cost you just as much or more as you'd pay for a laptop instead.
As for upgrading; if you have an external soundcard like the hamerfall, upgrading would be getting more A/D and micpres. Memory is more expensive, but it won't kill you. Another HD would have to be an external HD. Which would be more carrying around, but if you have a big HD fitted, you can record on that, leave the external drive in the studio and move it there when you get home. Not that much extra trouble.
If you're gonna upgrade your processor etc, you don't have any options with a laptop. But once you get a decent laptop, you don't need upgrades right?
A laptop is handy for more things btw. Just take it along and you can show stuff you've recorded to other people. Take it along and bounce files over the network, ... With the prices of laptops these days, it's really coming down to what you are planning to do with it. If you cannot think of a situation that requires moving our rig, go desktop. Otherwise, I'd consider a laptop right away.