I've played both S.S.E., and S.C., and once I got used to S.S.E I find it a lot more enjoyable and challenging than Supreme Commander. AND PERFORMANCE WISE SINS IS TONS BETTER, even on more modest systems, SINS runs fine. I don't have an ultra-high performance computer, it's pretty high-end, but it needs a lot of improvement to run SC at an enjoyable pace.
Stratagy-wise, I also like Sins over SC, as there are more choke-points, which helps check large fleet movements. Battles within a solar system can have mulitple fronts, but once you've pacified a star-system you can base your fleet defense around that solar-system's star to give unwanted intruders a warm greeting. Also, planatary defenses can be built sufficiently to protect your worlds, allowing you to use all of your fleet (if you desire) in offensive manuevers.
The AI also addapts to your stratagy quickly (even on easy) in SC looking for weak spots in your fleet's and planatary defenses tatical capabilities, making it challenging to overcome your enemy, but not impossible. The AI on supreme commander didn't really do anything for me, it's way too easy to beat the way I play, and I think that game can only really be enjoyable playing against another human player. I like it when I wipe the floor with someone in a skirmish, only to come back again to see they've adapted to exploit my weaknesses... it forces me to rethink my stratagy and it balances everything out--making victory that much more enjoyable.
HOWEVER, It would be great if SINS improves its diplomacy, it's on definitely on par with Supreme Commander, but definitely not on par with Master of Orion, or Dread Lords. I want to see my enemies diplomates, not work a quick menu. The people who are making this game, have, and can do better.
ANOTHER great feature would be timeflow control. The normal speed the game is set pretty good, if not perfectly. But, I am a macro-manager type at heart, and I really like to take a few moments every now and again to make decisions, and play with ideas, read benefits of tech-tree items, ships, structures, etc--not having to worry weather or not an enemy fleet is invading my space having had the time to amass a huge armada while I was simply tinkering with something. (That's one of the reasons I love total war in that it combines real-time and turn-based stratagy--this has the potential to do that in a more interesting way.)
The last killer-app feature this game needs is individual ship-class customizations (both in how they look, and what they carry.) This was the BIGGEST selling point for me concerning Dread-Lords, and will continue to be a draw, if they use it in this game--because, it, along with a timeflow (or auto-pause feature) would keep me entertained for hours and hours. Like I said before, the people who are putting this together have and can do better.
GRAPHICALLY the game is great! The ship-designs you're stuck with are extremely well done, along with structures, and planets--hell, there's even star dust, and when you're in the middle of a star-cluster space is a really beautiful place to wage war, or manage your empire. I wish that star-ships would manuever more during combat, without being commanded too, however, with weapons guidance systems as they would be amongst a space-faring war-fleet, I can't really fault them if they choose to fight in standing columns like a bunch of civil-war soldiers arm-in-arm. It just takes away from the 3-dimensial aspect of space, and falls short of the feel of space-combat that great scifi films and tv-shows bring to mind. (Manueving does, however make since in this game, since some weapons are big and directionally aimed, rather than guided in route.) Of course, that might slow the frame-rate down--but I really doubt it. I haven't been able to get this game to slow-down yet, except for when it auto-saves, (and it does this a frequent pace, which is good, because this game moves at a pace where you really appreciate it! Despite graphically outclassing all space-stratagy that have come before it this game seems to be incredibly stable, and very lite--even in larger systems--so me-thinks they innovatively compress everything you're not immediately looking at... after all, when your zoomed in, you're only looking at one planet's system, and not the entire solar-system, star-cluster, or galaxy. In Supreme Commander everything that's on the map seems uncompressed no matter where it is, or what it's doing.)
Overall, the game is missing a lot of golden aspects of Dread-Lords (including its sense of humor, random events, and alien personalities), but it's a lot more fun to play than supreme commander, especially on systems that DON'T have massive ammounts of super-RAM.