I played CItadels and Dominion many times. The gameplay is completely different - that has been already said. But it´s Citadels I enjoy most of these two. The reason is, that it´s more offensive type of game than Dominion. In Citadels you have to make correct answers to your opponents strategy, you are defending yourself against Warlord, Assassin and Thief or using them to your advantage, attacking with them. Dominion doesn´t have so much interaction between players, and since I prefer monster killing themes over citybuilding themes, I would just go for Thunderstone, which is very similar in game mechanics.

Shannar said:

Honestly I don't think Race is similar to either game.  Each of the three games is very different, Citadels is very family friendly and easy to learn compared to either of the others.  The strategy for Race would be very confusing for non-gamers and my personal opinion is that the vanilla version (pre-expansions) was very poorly balanced and involved a great  deal of luck (expansions have fixed this some, but its still a lot more luck-based than Dominion is).

 I have both. My wife and I both like Citadels better. I do agree that they should come out with some more expansions for replay value. Dominion is time consuming yet fun. It is more fun with multi people as my wife and I found out. Citadels is a great two to three player game that is somewhat quick. Overall, these are two great game from different companies. I'm happy to have them both and plan to teach my 2 year son both when he gets old enough. Right now i'm buying 1 to 2 board game a month for future use (teaching my son). I have already bought thousands of dollars worth of books for him to learn how to read quick. Now is board game time. 

I play online Dominion with friends and enjoy it quite a bit, but I've never played Citadels. It's now in the top 20 of dice tower so I came here to have a look.

 haha, quite funny i see this thread now.

I used to play Citadels a lot (a game I could even convince my sisters to play with me) and wished for a new game on christmas (cause my sisters never know what to get me)

and I chose Dominion, as I explained it to them, one said: it sounds like Citadels.

And I strongly disagreed cause I don't see any similarities (besides the medieval time of course): 

but she said Cards, Gold, You Buy Cards and Take them in your "deck". I know you can't compare them but for a person being not into games too much, they might sound similar

She will probably completely disagree after we played Dominion I guess