I have a friend that has both cable and satellite both with HD. He thinks they are about the same. The reason he has both is he likes the cable broadband internet and the extras offered by satellite. He is a hard core videophile.
All I will say it that "Digital Cable" is not HD. You will need digital cable in order to get cable HD, but the basic digital cable service is just a cable TV signal sent digitally.
Digital cable just has a lot more features and a lot more channels than regular cable, including the ability to get HD if offered.
You may see some improvement with digital cable, depending on how good your standard cable signal is.
I only mention this because I've had neighbors get digital cable and then wonder why the picture looks about the same.
I believe the same can be said about satellite digital TV and satellite HD.