Depends on how you want to work. If you're fine with sitting in a fixed place at home at a desk with a semi-permanent set up go with a desktop. You can get a large desk, spread out multiple monitors, have references handy, etc. If you want to be mobile, even if that's just going into the living room instead of holed up in a bedroom or studio a laptop is what you want. Personally I use both, though I retired my gaming laptop and picked up an Ultrabook. If you're wanting to buy a laptop keep in mind you get your money's worth and a $300 Walmart special is that cheap for a reason. (Though I can't really recommend most Ultrabooks for graphic art creation either.)
I've been using laptops exclusively since 2005, when Katrina trashed all my stuff. My current machine is a late 2010 Air and it does everything I want to do just fine.
A couple years ago I got a 24" external monitor to plug it into when it's at home. The computer hides under the desk and talks to the Bluetooth keyboard and the tablet I have on the desk; when I want to leave the house I can very easily unplug the computer and throw it in my bag where I keep the smaller travel tablet.
Admittedly this costs more money than just getting a tower and leaving it there all the time would have - but I like being able to take my machine out to a coffee shop or a convention or whatever without major hassle.
I think a PC is the best especially if you want to work with large files in photoshop and any other program that requires alot of GB. PCs last longer also, (I have had problems with dell laptops and the charger cord ripping all the time). Though there are times where I just want to lay down in bed and draw, but can't since my desktop isnt wireless. lol
Desktops in general are more reliant than laptops. This being said, I used a laptop for most of my life and it functioned fine. The only reason you would ever NEED a desktop is if you planned on diving into the world of rendering digital images or video, which I'm not quite certain you are. Buy a nice monitor and make sure your laptop is new enough to handle photoshop and you'll be fine. You'll never know you're not operating on a desktop, and you can take your laptop with you in the future.
Also, just because it's new doesn't mean its mandatory. Cheap 300 dollar laptops from walmart have the specs to run the previous version of photoshop. You should look into that. It'll save you money.
People have this idea that "I must get the NEWEST Wacom Tablet (when they've been nearly identical for the past three generations now. Anything above 500 pressure sensitivity is more than most people will ever need), the NEW Adobe Creative Suite (when Photoshop has only really changed to aid people actually working with photos), and THE BEST COMPUTER MONEY CAN BUY BECAUSE IT WILL HELPS WITH ART (Even though, in all honesty, digital painting, for most people's casual purposes, requires very little of a computer).
Desktops are practical for people who work at home or for excessively long periods of time, like me, and need the power of a workhorse to save images and have them ready to upload in a matter of seconds for work or school, but even I get jealous when other people have the option of bringing in their entire computer rather than transferring everything to a hard drive when they want to work on something outside of home.
What are you using this for? We can sit down and discuss what you want and maybe save you a few hundred dollars. The PCs you're thinking of when you're thinking of computers that can handle BIG files are the computers that will start costing you over a thousand dollars, so you need to think realistically about what you're going to be doing.
If screen size is a problem you could always purchase a large monitor and plug it in. I currently use a tablet laptop. Usually I start out sketching on the smaller touchscreen, then I plug in my large monitor and have a zoom in on laptop for detail work, and another window open on the large monitor so I can see my whole piece. Now when I work in vector I plug in a mouse and a keyboard and basically use the laptop to run Illustrator.
I personally enjoy the laptop because of the portability. I can sketch out in public, outside or in a different room of the house. I don't have to be tethered to the office.
I'm using a PC... Laptop are expensive and with a long record of poor reliability. I prefer to buy all the pieces and build the PC by myself. Of course the screen size is a critical element, I can't imagine working on a 17".
I think that the specs of a PC are more conducive to digital art and the power that the processor needs to run the software etc, but unless you buy a pretty old laptop it should have the power to run whatever you want to do. For reasonably fast and responsive digital work all you need is a dual core processor and 1 gb of ram.
I use a laptop, because I like to take it outside and to other places with me to work on; inspiration strikes anywhere.
But you could probably do more with a PC. Just some opinions; I'm sure you'll decide on what's best for you.
thanks a lot for the reply ;] well , I have a dual core processor and 4 GB ram, but when I'm working with many layers or I'm using a more complicated brush, I can really feel Photoshop is slowing down... very irritating, especially when ispiration struck me