This probably puts me in the minority, but so far I've tried to avoid researching higher marks of triangle ships whenever practical. Instead, I wait to see if I can acquire bonus fleet ships with overlapping roles and upgrade those instead. My reasoning is that (1) I can get Mark III & IV ships without spending 2500 knowledge on the Mark II upgrade; and (2) bonus fleet ships are intended to be 30-50% more powerful than triangle ships. I'd imagine the devs would be pleased I've chosen this strategy, as maximising variety
is one of the design goals.
But something just happened that may forever change my plans.
*MINOR SPOILERS AHEAD*
In the first exo-wave of a 9/9 2-homeworld game with Golems (hard) and Spirecraft (hard), I met my first Hunter/Killer. Part of what makes these beasts so nasty is their command-grade hull type. This renders them near-immune to most turrets (all but heavy beam cannons get a 0.1x damage multiplier), plus there are literally NO starships or bonus fleet ships that get a bonus against command-grade hulls. The only ship I've found that does is the humble Bomber.
I think that in any game with serious exo-waves, H/Ks will be one of the more likely ways I could lose. This means I'm all but certain to research higher marks of bombers, rather than hold out to see whether Space tanks/Chameleons/Electric bombers/Raiders/Bombards become available.
So, I'm wondering whether there's any special reason no bonus fleet ships get a bonus against command-grade hulls? I ask because giving a triangle ship a niche that no bonus ship can meet, and therefore encouraging players to always upgrade a particular triangle ship at the expense of upgrading a bonus fleet ship, seems contrary to the design goal of maximising variety.