It's not quite as black and white as to the capital ship choice in SP games as Jixra makes it out to be Colonizing capitals make it easier to expand faster, but that also mean they make it tougher for new players learning how to juggle between planet upgrades and building ships in the beginning, and make it easy to over-expand early and not be able to defend or worse yet, not having any money income.
In any case, with Easy/Medium AIs (what a beginner should be attempting to play against ) speed of expansion is far from very important, so I would actually recommend ships with more combat punch: Marza or Kol for the TEC, Vulkoras for the Vasari (not a huge fan of the Kortul by comparison, but it's good too as long as it has antimatter), Radiance for Advent.
I think Advent's Progenitor mothership is the only colonizer that's all around great to have in a combat fleet, due to its shield regen. The Akkan does help with range, and the space whale is good against single ships with its armor/DoT ability, but the Progenitor keeps a big group of ships alive for a long time.
Most of it boils down to preference, but I'll admit playing against lots of Unfairs, I go with a colony capital first.