Hey there,
Regarding which is the best, check the archives here for comparissons as there exists alot of info to trawl through.
As for what the assesments consist of, you are best advised to ring up each of the schools that you will be attending and asking admin to send you out an email regarding what each test entails. Oxford sent me alot of material I have since discarded as I chose not to go there, and BAE have told me that their test consit of the following: Co-ordination tests, spatial awareness...etc, verbal reasoning and numerical reasoning...all very similar to the process big companies use when recruiting graduates. If you head to Waterstones, or similar, you can pick up the 'how to pass numerical/verbal reasoning series to practice before each assesment.
Thats as much as I know, I am due to sit mine soon, so hopefuly someone else can provide a more detailed structure?
Good luck!