I have been thinking about picking up CoD4 again. I mostly want to play the campaign again because I completely rushed through it, as I picked CoD4 up the week MW2 came out and then traded it in towards MW2 after finishing the campaign. Having not played a FPS extensively since Halo (maybe?) and not played a FPS extensively online at all, I wanted to see what I was missing out on with CoD4. It was rough at first not having played a FPS in quite a long time, especially the entire hour and a half I spent playing CoD4 online. I did not find any kind of FPS stride until well into MW2. Now I think I would enjoy CoD4's campaign much more than I already did when I played it, as my skills have improved some and I would not be in as much of a rush to finish it as I was before.
Considering how little time I have had to play Black Ops, it may be tough to find time for CoD4 though.
I can give you little feedback on Black Ops outside of the campaign, as I have only spent about two and a half hours playing online. The campaign was fun and the trip jumping through years was pretty cool; compared to CoD4 or MW2, it was probably about the same for me excitement and entertainment wise, which is to say I very much enjoyed playing through each games' campaign.
On the multiplayer side, I can only really tell you about Combat Training. It is like playing online, but instead of people, you play against bots. I have found it to be helpful in learning the maps before jumping into multiplayer, as it gives a bit of an objective to exploring the maps, instead of just wandering around not paying attention to the map's features. In my little time playing against people, the Combat Training has paid off, as I have not been a complete noob wandering aimlessly exploring the map and not paying attention to people shooting at me.
Lastly, since I skipped WaW because of when I started playing the CoD series, that is basically starting at MW2, I cannot tell you much about the Zombie experience, as I cannot compare Black Ops to WaW and I have spent about half an hour playing it so far.