If you think Medieval 2 was a "graphical upgrade to Rome" you didn't play it for very long. The battlefield strategy was entirely different as were a lot of empire-level features. Try going back to Rome after a few months with Medieval 2. It's hard to do.
decemberscalm has the right of it -- Medieval 2 is indeed little more than a medieval skin of RTW. It had the same dumbed-down combat system as Rome, and the same dumb AI as Rome. It's mostly style over substance, just like Rome. All of Medieval 2's extra bells & whistles can't cover that up.
But back to the topic at hand:
I'm not a beta-tester for Sins, so I've obviously yet to play the game. But I *do* have Stardock's Galactic Civilizations 2, and I've gotta say they blow me away.
The game itself is awesome, but what I really appreciate even more is Stardock's approach to their work. They continue to provide free regular updates -- not just patches, but actual additional content -- months after a game is released, and they interact with the community constantly. (Indeed, there are times I wonder how
Frogboy,
Yarlen, and the rest ever get any work done!

) In relation to that, they're also very responsive to requests/critiques/etc fans have of the game, and do an amazing job of responding to said requests & critiques. They're always wanting to make the game better for us.
Now Stardock has teamed up with Ironclad on Sins, and for good reason. Both companies have similar philosophies about how to make games -- and just as importantly, how to treat the gamers themselves. They realize that the best way to keep the general fanbase happy is to provide the best game possible while still doing so in a fairly reasonable time period.
So given all that, of course they aren't going to release the demo until later. Sure, I admit that kinda sucks for me personally (as I like to demo games before buying them, no matter how promising they are), but to hold that against Ironclad would be incredibly foolish and short-sighted of me. They feel their responsibility are to those who've already purchased the game (or soon will be as soon as Sins hits the shevles in the next day or two), and I have no problem with that.
It's usually only those who always want to play the newest game "right now!" that complain about the lack of an immediately-available demo. And to be honest, I strongly sympathize with that type of gamer -- more than you know! To those people, however, I tell you that patience really is a virtue! Yeah, the saying is cliche, but it's still true; it was a lesson I learned the hard way.