In the specific case of Transformers though you wont need a comparison site as :-
The 2007 Transformers is only on HD DVD
The 80's cartoon version is only on Blu Ray.
As for sites, as previously mentioned, I use HighDefDigest.
From reviews I have looked at for the majority of films that are on both Blu Ray and HD DVD, the video quality is likely to be near identical and only the potentially differing audio standards may sway your decision.
As for extras, well personally I havent ever watched all the extras on any disc I own, Blu Ray HD DVD or even SD, but thats just me.