That is strictly a personal choice and just depends on your priorities. Some people could care less where they go, as long as they are on a ship. Others care more about the itinerary.
For me, the itinerary is the most important. I figure out where I want to visit and then choose the best ship for me that will take me there. Others do the exact opposite.
However, I do find that the more sea days I have, the more weight I put on the ship. If I just have 1-2 sea days during a 6-7 shorter cruise, I don't really care so much about the ship, since I won't be spending a great deal of time on it. Once I start to hit 3 or more sea days with fewer ports, then I examine the ships a little more closely.
Good luck and have a great first cruise!