I have one of each and they both have benefits.
The biggest advantage of a keeper league over a dynasty is the draft. I love drafting. A live draft with a group of friends, with the beer, food and music that goes with it, is a hell of an experience. A Keeper draft will run 15-20 rounds usually, meaning you can count on at least a 5-6 hour draft that includes most of the players in the league and a lot of good trash talk over picks. Meanwhile in my dynasty the draft is 5 rounds, mainly of rookies, defenses and kickers (we keep 18 from year to year and most of us dont carry Kickers during the offseason). It takes an hour or so and isnt nearly as fun.
The dynasty league does allow more more long-term strategy decisions, and rewards investing in sleepers, since you carry more depth. It is unfortunate that the bottom couple teams have a hard time moving up from year to year, and when we started we lost two players who didnt see themselves improving any time soon. Its easy to get discouraged when you are keeping Kolby Smith... In keepers usually every team has 4-5 keeper worthy guys.
Ill say this though, teams are a lot more willing to make dump deals in dynasty. While in a keeper its hard (as a bottom feeder) to get someone else to give up a keeper-worthy guy for a couple of vets it seems to happen a lot more in my dynasty. Someone might be willing to give up valuable pieces or future studs for temporary help when its one of 18 keepers versus one of 4-5. Also makes it worthwhile for a bad team to take some risk on some future upside guys late in the season who would never be kept in a keeper league. For example, in 2006 I grabbed LenDale White and Kellen Clemens off the ww near the end of the year as sleeper keepers in my dynasty, and both paid off for me last year (obviously mainly White). I also swapped an aging Terry Glenn to the eventual champion for Brandon Jacobs, who was a bench player for him in 2006 but a starter for me much of last year.