Well, Kris, you're in luck - I use them both.
Depends what you want and how much you can/want to put into it.
Storefront is a fairly sophisticated out of the box solution. You can get a functioning site complete with Affilate scheme up in a hour or so, once familiar with it.
Its also open sourced code so you can redesign it...BUT you have to get to grips with code. It is misleading of LaGarde to call it an extension. The set up and admin is hardwired but if you expect the design elements to show up in UD or FP for that matter - forget it. I don't know the UD variant, I started with FP one (the original). This one is actually a bit better than the UD, funnily enough. $279 to $679 (I think) plus £129 for the Merchant Tools.
UcartII: written by Joe Scavitto who is a C man. Runs a family chocolate store. He started by updating the original Ucart (free still from Powerclimb). Then he rewrote the whole thing for UD4 and added bits (stock control, shipping etc).
This is TRUE extension. You can use the UD stuff to the full. Unlike SF you get the full visual display in design mode. Very flexible. Steeper learning curve. $99.
You could always download UCart1 and play with it. UC2 is much more sophisticated and powerful, but you'll get the idea.
On line procesing: yup. SF comes with an Authorize.net COM object built in.
There is UD Extension for AN you can use for UCart(s).
So, in summary they are slightly different beasts.
If you want to know more and can cope with my typos, give me a shout :)
BTW, I assume you are talking ASP on Win. ASP on Linux is a no-no for SF and UC2, but UC1 works.