I would definitely recommend Belfast over Dublin just for the fact that it is so much friendlier than our capital.

Dublin is a fast paced faceless city that really hasn’t anything of significant interest to see and definitely doesn’t live up the stereotype of Irish hospitality.

The noted sites in Dublin are things such as the Guinness Brewery(like any other brewery in the world), Phoenix Park (like any other city park in the world) and Temple bar (the inner city pubs and clubs – like any city centre in the world).

The Giants Causeway is beautiful and unique in the world and although you may be hard pressed to reach it in such a short time if you stay in Dublin it would definitely be a worthy (long) day trip from Belfast.

Donegal (a day trip from Belfast) is the most beautiful corner of Ireland in my mind, if its not too rainy even the scenery on the drive there is worth the trip.

Belfast or Dublin that is hard to deside I think each one has there special character. Well if you go to Dublin and are a fan of modern architecture you should have a look at the Spire (build 2003). If you like nightlife you should go to Temple Bar it is famous for it's bars.
And Dublin Castle you must have seen it is so beautiful. If you are a romantic go to Half Penny Bridge at night it's great. In Belfast you should visit the beautiful castle, the theater and the Ulster Museum if you are a fan of recent history go to the library as well. They have an exponation of the IRA Conflicts there.

If you are strapped for time, I would suggest Dublin. I was in Belfast and we did go to see Giants Causeway (breathtakingly beautiful seriously) but it is a trek for 2.5 days...

In Dublin everything is right under your nose... if you like art I'd suggest National Gallery... if you like shopping, St. Stephen's green shopping center...and as for pubs, you will be spoiled for choice, but I'd suggest the Dame Tavern a little bit off Temple Bar (great mixture of locals and great live music)... oh and dont mix Guinness and tequila, not a good combo :)
Whichever you choose have fun :)

i would recommend dublin. i completely disagree with vincemcc79. i thought dublin was extremely friendly...everyone i met was helpful and courteous.

personally i really liked dublin because everything is within walking distance. anywhere you would want to go is immediately there for you rather than worrying about taking buses or spending money on taxis.

i also love the way the city is set up and it is ideally perfect if you only have a couple of days. when i visited dublin for the first time i spent 2 and 1/2 days there and it was the perfect amount of time to see everything i needed to as well as get a good idea of the culture. it impressed me enough that i went back for a second trip.