SC2 vs C&C
Star-affinity is absolutely right about it being a similar kind of game - but SC games have a very different flow when compared to C&C. I've only played a few Command & Conquer games, but having played Blizzard games my whole life, I had a very hard time adapting to the style/speed of C&C.
I didn't get into C&C very much for just that reason - but I think I'd of had an easier time learning SC2 after playing C&C for years, opposed to trying to learn C&C after being spoiled by Blizzard games.
Granted, there is a lot you have to worry about in SC2, the C&C interface always seemed very complex to me. I had trouble remembering what each building was for, and where certain units were produced. Since I had such a hard time with the general interface of the game, I never got to dive very deep into strategy.
SC2 also has 3 races, which are completely different from one another. From what I've seen in most C&C games, each race has pretty much the same units - except race A will have a stronger version of infantry, race B has better mechanical units, race C has better air... etc.
I would say Starcraft 2 is fairly easy to learn, especially if you'd had practice and/or success with similar style games.
As far as purchasing the DVD's or the digital download - that comes down to preference. Blizzard's system for digital downloads works very well, and is very easy to use. Once you purchase the game (by either method) and register it with Blizzard, you will have access to the game installer via download, wether you bought the DVDs or not. Benefit to having the DVDs is a) a hard copy and b) ability to install in the off chance you have no internet.