I think Steve has the right of it: compared to ships STOs are logistically easier and less costly to maintain. Not mentioned by Steve, but also relevant, is that STOs are produced by GU training centers rather than shipyards or factories, so the use of industrial capacity is different and potentially more (or less) flexible.
I think the comparison to STOs has to be with orbital platforms, as both of these are basically the two C# branches from the old VB6 PDC concept, in fact one could almost roleplay an orbital station as a PDC if one wanted to. In this case the comparison I think comes out with a good balance: STOs are more complex and perhaps not as efficient per ton or per BP (as noted the uridium costs can be substantial), but are logistically less demanding and in many cases are much more difficult to dislodge. Orbital bases on the other hand are likely to be more efficient and flexible (notably able to mount missiles, but can also be tugged to e.g. defend a JP). However you want to rationalize the gameplay limitations in terms of RP (for example, I could say that STOs are geographically dispersed and not connectable to a single BFC, and gathering them all in a single facility like an old PDC is better modeled with an orbital base - but your headcanon may vary), in terms of gameplay balance and presenting with multiple viable options I think the tradeoffs work well and I cannot find either option to be strictly superior
Comparing to typical warships on the other hand is really apples to oranges, as these have significant "inefficiencies" from a planetary defense perspective due to the need to mount propulsion and other necessities that an orbital base does not worry about.
As for Gauss vs Railguns, Garfunkel has it right as he has stated the age-old wisdom - Gauss is for good PD once teched up, railguns are for cheap PD at all tech levels. The balance is tactical vs strategic benefits.