I really like Citadels and don't care for Dominion at all (I think this is counter to most people's postings).
Citadels is a deduction/subterfuge game. Dominion is just a deck growth/point acquisition game.
I find Citadels has endless replay value, like a medieval RPG-influenced Texas Hold'Em.
Dominion... I don't even feel like I'm playing against my opponents. I just look at the chosen deck, and decide what I'm going to pick immediately with minor adjustments if other players lean in one direction or another. I feel like you could make a spreadsheet (a fairly involved one) for most Dominion line-ups and win. It's strange because a number of the people I play with love Dominion and I never want to play it, but I almost always win.
On the other hand, I love Agricola, and you could make some of the same arguments about that game but I find the resource-taking to be much more competitive and your play has a much larger impact on your opponents. It's interactive where Dominion just really isn't. In Dominion, there are tons of cards available and your strategy isn't usually ruined from 1 or even all your opponents taking a card from your desired next pile. However, it's worth noting that I haven't played with most of the expansions if they make a huge difference on the gameplay.