It really depends on what your wanting to do. Rainbow reef was great, but when you get a DM you are in a group. When you go with Horizon you hire a DM for you personally and the dive is controlled by when you are ready to go up, not when the group is ready to go up.
In my experience I have had longer dives with Horizon. Rainbow is great and I like everyone but the dives are a bit shorter, they are more conservative.
If you have a buddy with you and you go with Rainbow you do not have to go with the guide so you are not limited by others in the group, same with Horizon.
Hope that makes sense.