I would say Forza runs a 3rd to Enthusia.
I have been able to quantify (in my opinion) the major difference between Enthusia and GT4.
I use the Ruf CTR Yellowbird as the car - and the Nurburgring as the track.
Back to back Enthusia vs GT4 sessions ahd me lapping GT4 with stock Yellowbird at about 7:48ish - Enthusia I struggle to get below 7:54ish.
The made difeference is the minimal tactile LATERAL forcefeed back with Enthusia.
But not to say that it is less "realistic" than GT4, it's just that the very firm tactile lateral feedback allows you to corner a lot more agressively and "feel" the caras it is about to let go... almost the way you get feedback through the wheel in a real car.
This tactile lateral feeback is what gives GT4 the "bumpy track feeling".
It almost seems as if the tyres in GT4 offer way more grip, but what is really happeneing is Enthusia feels a lot like nion-force feed back wheel on the XBox to you end up driving like a movie actor, sawing on the whell left-right-left-right until you get the feel for the nuetral nature of the wheel - even though the car you are driving is a vicious oversteerer.
This all means you end up providing your own forcefeed back.
It also means you can hang some wicked Walter Ruhl style slides lock to lock fishtailing though sets of s-bends.
I am still undecided as to which is the over all most realistic - GT4 almost seems to allow yo uto lap too fast under the same circumstances.
BTW, GT4 stock tyres let me lap at 7:48 - only switching to super-soft racing drops my time to 7:15 with a couple sections I can probably drop 2 seconds a piece in.