I did both this year and it was my first time at COTA. Both are great tracks to spectate and the area is fun to explore in the evenings. Monterey is just awesome to check out. You have to go to the aquarium and explore cannery row. You'll bump into some of the racers and teams and have your choice of awesome food everywhere. Sykes was down there drinking a bit too much lol. The energy in that area is incredible. At the track, WSBK is a lot more laid back with the security. They do the press conferences in a big tent in the pits and you can walk right in, that's pretty cool. You'll see most of the WSB racers walking around talking to people in the pits like it's a club race, it's just more casual. COTA is really cool too and Austin is fun. You have to go to the Handbuilt Motorcycle Show it's awesome. Plus they set up a wall of death which is just about the coolest thing you will ever see. Honestly it's hard to pick a favorite, both tracks are awesome and Austin and Monterey are equally as fun to spend time in.
My wife and I had a great time at the SIlverstone WSBK race. We stayed at Oxford and the drive in to the track wasn't too bad. If you extend the trip a little more, you could catch some of London too.
Have a look.. http://www.polepositionvip.com/index.html http://www.grandstandmotorsports.co.uk/bigrock/jerez-motogp.html
Nothing - nothing - compares to the electric atmosphere at a MotoGP event in Europe. Watching it on TV is a pale shadow. US rounds are tepid by comparison.