I don't have anything scientific to say but as a rube user nicotine and caffeine seem obviously different to me. I don't even really "feel" caffeine. I don't experience cravings and I don't experience withdrawal. On a day to day basis I just notice caffeine helps keep things ticking over better. So I drink four cups of strong coffee and two cups of tea.
And by caffeine I mean tea and coffee, which we all know are both far more than just caffeine.
I only crave tobacco (tiny snorts of Dholakia White snuff in my case) when under stress and short of sleep. It's insanely valuable for pulling long hours and mustering concentration when you wouldn't otherwise be able to manage.
On any given day I will always enjoy a good coffee or tea. The idea of using tobacco on vacation doesn't even cross my mind. For me it is very clearly a substance that helps to cope with short term stress.
You don't "feel" caffeine because you take it so much that you've developed a tolerance to it. If you raised the dose or took a long break you would feel it. Nicotine works the same way.