Sticking solidly with 5/5. Equal strength because if we fail to take both we're going to be quickly stuck in a massive and unpleasant campaign.
As far as strategy goes, we're going to need a turn to bring back and reload all transports (Can the budget transport make the Spire-demispire transit fast enough to load and drop off one squad of marines per turn? Could a normal one?) before sending our budget transport to another demispire and sending the large transports out to reinforce the already-being-captured Belt Spires.
Draignean, will we be designing SHIP tactics, or only ever designing INFANTRY tactics?
Every ship can make two jumps per turn unless specifically and explicitly noted otherwise. However, since there's a lot of quantum time to deal with, it may arrive conceptually later in the turn. Thus a fleet of 'fast' ships may arrive and secure an area/disembark troops/etc by the time a 'slow' fleet shows up.
Anyway, as to rate at which you can deploy marines, lets assume a move speed of two. A loaded transport can move to a location and drop off units (move and action, or two moves and action). Then, next round, it moves back to the spire and reloads on Marines (same pattern as above). Rinse and repeat.
So, if a spire is within two jumps of your Home Spire, you can run a single full transport of marines there every other round. If it's greater than two jumps away, that falls to once every four turns.
Ship tactics are definitely a thing that you'll need to keep an eye on.