I did a quick comparison between a bare bones engineless no maintenance "fighter" with a 10/C3 railgun and an STO-PD unit with the same gun and sensor tech, and the ground unit seems consistently cheaper in terms of minerals and BP cost (and size of course), plus it has better chances to hit (listed anyway).
I have no idea how they would work in combat, since the chance to hit a ground unit is calculated differently than a ship and that's still beyond me. I suspect the STO will do better there too, since it can fortify to lessen its chance to be hit. There's also training (crew and ground unit bonuses) to consider, as well as commander bonuses, the fact that ground combat is kill/no-kill vs incremental damage like ships, etc. That's too much math for me!
The orbital platforms are more flexible in design however, and if you make them actual ships you can refit them with better tech as you go along if you want. A lot of it depends on your playstyle and how much micro you want to do.