I'm probably going to buy a Shuttle myself, so have thought about it: laptops come pre-built, so you get more for your money w a SFF because your own labor is not part of the price. Laptops are much lighter & easier to carry around--SFF can go to a party, but not to carry all around every day like a laptop--plus they only run on AC, not batteries as far as I know, so you can't use SFF at all out and about on the train etc., even tho for example the SB65G2 model comes with built-in WIFI.
If you want a big screen you can get one with SFF since only the barebones system is included (case, PSU, mobo, cabling).
both will limit you on optical drives & second HDDs, but in SFF you can put a second HDD in the external 3.5" bay which would normally be the floppy drive bay.
You get your choice of all the cards w the SFF--any compatible graphics & audio cards, tv tuner cards etc etc.
You can customize a SFF with fan lights, plexiglass panels, colored shrouds, your own paint job, or any other case modding--can't really do that w a laptop.
I've never heard of anyone overclocking a laptop, but I'm too much of a newbie to really know if that's possible.
If you really want to do multimedia, why not wait for the ABIT Digidice SFF, which has just been or is just about to be released to market? Look it up at sfftech.com or elsewhere--it is the ultimate multimedia 21st century small computer!! hot keys for every media function, plus room for 2 opticals so you can burn w/o ripping to hard disk with one touch of a button, PLUS all the overclocking options made much easier too.