I personally think Garmin's touch screen interfaces are going to be a lot better than anything designed for PC use. When software developers created GPS solutions for laptops - I don't think they designed them with being used by a driver with the vehicle is moving in mind. Everything I've seen has too many drop menus and small fonts to really be effective while on the move, although there may be something better out there.
I'd keep the 2720 - and maybe if you decide the Laptop idea is working well enough on it's own - THEN get rid of it.
Although I'm not so sure how much you rely on a GPS. When I'm in the mountains, I can easily navigate with just a map and compass. In the dunes it's much much harder to triangulate your location, as there won't be any visible landmarks on the horizon. To you a GPS might be a luxury, in which case it's ok to play around. For me personally, it's more of a necessity once we head out of the city, and the Garmin will probably be easier to use. I've used the TomTom GPS in England a few times and it's nice how simple they can be.
Also - as a Computer Science student I've dealt with computers a hell of a lot, and if there's one thing they do well, it's screw up. Errors, Crashes, Overheating, Lockups, Memory issues, Lags, all seem less likely on a single, contained, tested system. I don't care if I have to reboot my CarPC to get the MP3's playing again... but I do care if I have to stop, reprogram my destination, and start all over again. I think with that in mind, rather than ask the computer to multi-task, I'd prefer to have a seperate "computer" dedicated to just one task that it was designed for. It makes me more comfortable.
If I can find a spot for two 7" screens, a head unit, and a rediculous amount of switchgear, you can squeeze the 2720 in there somewhere along side the computer project

Problem is just all the toys get expensive!