A lot. Hegemonia might not have much diplomacy (but then again, neither does Sins, most people play Sins with locked teams just like an RTS), but Hegemonia is much more of a 4X, at least in my opinion since it has a much deeper research tree and you get involved with your planets more. In Hegemonia you can build all sorts of structures on your planets. In Sins you conquer, colonize, build a few orbital structures, and spam frigates. To be honest, I was hoping Sins was a cross between Hegemonia and Galactic Civilizations 2. I thought it had fewer planets, more ship-to-ship combat, and a bit more micro than it does now. Instead I got what I refer to as Supreme Commander in space, when I thought I would be getting an involving 4X game in real-time. Yeah, I suffer from a good deal of buyer's remorse.
Hegemonia appeared way too one-dimensional to me, with a quite linear upgrade path and a severe lack of diversity in unit types, tactics and strategy.
It offered neither the scope nor complexity that is a staple of pretty much any 4X game and i rather left the impression of some sort of "poor mans Homeworld" on me.
And while in Sins a simple "spam" strategy is possible there, it is also easily counterable and there certainly is a much wider variety of possible fleet compositions with ships that actually fill specific rules, including carriers, fighter defense, long range missile attack, anti structure siege torpedo ships, and a WIDE range of utility ships offering fleet wide targeting bonuses, individual ship repair, sabotage drones, et cetera and a range of capital ships with unique special abilities to top it off. Heagemonias unit selection is quite embarassing in comparison not just to Sins, but the almost any "decent" regular RTS i played up to including Homeworld and Supreme Commander, ... but i guess that goes to show how opinions differ
Imho. Heagemonia would be a rather lackluster example of a regular RTS game and that is what it was to me: An RTS game that happened to play in space, but sadly didn't offer much complexity in gameplay or strategy.
That it has a "Tech tree" and happens to play on maps that features a couple of planets instead potential resource/base locations doesn't suddenly put it into a new genre.
There are enough titles that offer a quite similar gameplay experience that "just don't happen to play in space".
And Homeworld would be an example just how much better and playable the basic idea of "classic RTS in space" can be spun.
Sins on the other hand has a quite different gameplay focus, scale and scope. It's gameplay on a different level that requires different thinking.
Again... that doesn't make it better or worse, not even more or less complex than other games ... but the different focus on gameplay with the emphasis on the strategic layer instead of other games tendency to focus on the tactical layer as well as the scale and scope of the decisions that have to be made and the odds that have to be balanced, matter of fact is what could be an argument to put it in a genre of its own.
I.e. i would argue again that there is a difference, and it's not a differnce that could be measured by pairs like "good/bad" "complex/simple gameplay" or even "diffcult/easy"... rather it's different... in what kind of gameplay it offers and focuses on.