Trident is the evolution of Dolphin, consider it as Dolphin 8, but because of the number of structural changes in it, we've decided to change the name as well.

There are a lot of differences between them, but the main are:

Dolphin Pro pros:
- a lot of build-in module
- a lot of 3rd-party modules
- native mobile app

Dolphin Pro cons:
- not mobile friendly (however it should be changed in Dolphin 7.2, but not as much as in Trident)

Trident pros:
- mobile friendly by design (including see more retina support)
- many structural changes which easies modules creation
- completely new modules with no alternative in Dolphin Pro such as Albums and Conversations

Trident cons:
- small number of 3rd-party modules available for now

Also please take into account that migration from Dolphin Pro is not ready yet, but when it will be ready it may not transfer all data, since some modules are very different or not available.