We took the
Disneyland Express last time we were there, and it worked out very well. If you book online, you can save $2 per person. This ends up being $13 for adults and $11 for kids each way. Kids under a certain age are free. I think it is 3 years or younger.
We have also used Super Shuttle, which is slightly cheaper at $10 each way. I will say, though, that the vans are not as nice and don't have nearly the room for luggage. Every time we have taken Super Shuttle, the van has been really full and the luggage is really shoved in there. We had so much luggage last time, there is NO WAY it would have fit in there along with all the other passengers'.
The Disneyland express is a large touring style coach bus with room for the luggage underneath the outside of the bus as well as above you inside. The big advantage for us was that our daugher was 2 at the time and would have recquired a car seat on the Super Shuttle, which meant hauling it down there on the plane and then taking the time to get it installed each way. No car seat was required on the Disneyland Express, since it has bus style seats. I am a crazy mom as far as safety is concerned, so I wasn't too sure about her riding without the car seat, but the convenience of it more than made up for my fears. Plus, I felt totally safe the entire ride, and held her in my lap the whole way.
The service was better on the Disneyland Express and the vehicle was MUCH nicer. For us, the couple of extra bucks for this shuttle was worth it.
Have a great trip!