Commander Level is a good time to test out an escort.
You will get an Akira class which is closer to a cruiser than a Defiant at captain level.
If you want to sample a good science vessel, I recommend the Intrepid when you reach Captain.
The Cruiser and Escort at captain are the fan favourites Galaxy and Defiant class. They are the most specialized vessels in their respective ship type, but they also have the most noticable weaknesses.
Galaxy can take a lot of damage but is also almost the slowest cruiser with the lowest damage.
The Defiant is among the hardest hitting and fastest ships with the weakest hull.
But for now, pick the Akira.
You will get a Commander level cruiser for free at level 25, if you play the mission Temporal Ambassador anyway.