Well, we don't yet know about GT4's physics, do we? There's what.. three people on this forum who actually have the final game? And they've said that the demos are no indication, that the physics were overhauled yet again before release.
Which means we don't know about the "ass-kicker", either. With the new physics engine and a 900-degree wheel, real donuts may finally be possible. We just don't know yet.
Enthusia looks interesting, but not really original. And it does seem to be more of a "GT Clone" than other "Sim racers" out there. A lot of the same tracks (Nürburgring, for example.. funny how every racing game has to have the 'Ring now, ain't it?), and nearly every car I've seen on the car list is already in GT4. Which begs the question of "why would I bother?" With a different game, I want something different. I want totally different tracks (and I much prefer real-life tracks to fantasy ones), and I want totally different cars. Enthusia doesn't look to be providing either.
For the "realism" argument that PD put forth, I think they're thinking something along the lines of Flight Simulator. I've heard many pilots say that playing that game is harder than flying a real plane (I'd tend to agree). Enthusia may be the same way. So in-depth as a "Simulator" that it forgets that it's a game. Which PD has never lost sight of.
I will be keeping a sharp eye on it, though.. but I'll also be reading the reviews before making a decision to purchase. If it doesn't hold up there, I won't bother.