It depends a lot on what you enjoy.
For me, grinding away to earn the cash to buy the ships I want is not fun at all . I do not have the time or energy to do that type of thing any longer. In my old EQ days maybe, but that was 20 years ago.
For some, the journey IS earning the ships. For me, the fun is all in the roleplay once I have the ships I want. Sure there are some ships I am holding off on buying until release but that is more because they are not critical to my plan. The ships I do own are part of the character stories I have created.
I have a smuggler, an explorer and a crack pilot that I plan top play on my first three accounts.
The others are reserved for crew and alts that I have in mind but have not fully developed their stories yet.
In Star Citizen there is so much to do, so much to see, so many places to go and people to meet, both NPC and PC. I do not want to waste my time grinding out credits when I could be exploring the verse, or sneaking past military blockades with refugees on board or defending a station from pirates.
Sure, one might say you can do those things and earn credits, and to some extent that is true. But you can do all of those things and not be concerned about the credits when you are already in the ships you want.
The answer to your question, should you buy the ships before or wait to get them in game, is not a question of having something fun to do in game, it is a question of WHAT is fun to you.
If earning credits and working your way from nothing all the way up to a Carrack sounds like fun then you have your answer. Likewise if that sounds like a chore in the way of allowing you to spend all of your time exploring the edges of the verse in your Carrack, well then you have your answer there as well.