Depends if your signed on and your requirements? Is it on the full GP circuit? This makes a big difference as to the best shots. I've seen some belters from the Maggotts, Becketts, Chapel area (its the area that they change direction in with a grass run off area) on a 400 shot loose. The grass had stripes cut in it following the track. Stunning!
If you need the normal Silverstone start/finish line shot down the straight from copse then I've seen both 400 and 600 used and both are fine.
A 600 is good for the shots back up Farm straight from the Bridge. Nice and tight!
The others, like Vale, Abbey and around Luffield/Woodcote a 400 will be a bit tight, which is cool if the racing is close but if (when) they spread out it will be a one car only type of shot, if that makes sense. Depends on your requirements.
So choice of 400 or 600, how far will you be walking? Which do you find more comfortable panning with?
If it was me, 600 (I wish) on one and maybe a 300 or 70-200 on another body.
Have fun,
Shaun