Sky do not offer Channel 4 on any of their packages, so it is probably not an option for you at all. With Sky Digital, you will also need a dish, but in my opinion, it is a far better service than NTL. Because of the dish though, it is available anywhere, provided that you are not in an apartment complext where you are not allowed to install a dish.
It is about 3 years since I had NTL Digital, but I was constantly having problems with dropped signals, slow to change channels, slow menus, customer service was a joke, etc. Even still, my parents have had NTL Digitial for years, have Sky Sports, Premiership Plus, all the Sports options, and they have never had a complaint. And I'm sure they've improved since then.
But id you've used both, I'd guess most people would say that Sky is the better service...provided that you don't need Channel 4! You do get E4, and More4, but no Channel4.
Some areas have specific companies that only operate locally, so depending on where you are, that could also be an option to you. I know of a company called Century where I am, but it is basically Chorus with a new name and Sky Sports1 thrown in for free!