Not familiar with their models Muddy. I have a SAC bag. But I'd get the biggest one available. Travelling 2-up, you'll need all the room you can get, especially with those oversized glove compartments they're calling saddlebags on the SERK 3/4's! 
Hoist! 
Thanks again Hoist! yea, I can relate to the oversized glove compartments

. Have ya ever tried to get a packed liner in or out of them boxes?

I can just see the liner in another year....they'll probably look like a kite tail!!
2pak: Missed ya... we live approx. 12 miles from Hollister! Looks like your timing was good too...as the Yosemite fire is getting real close...sounds like you had a great ride! Thanks for the info!
sadunbar: Looks like a great deal, but I need a bag for the serk...hopefully somebody will chime in!
Lilbear: Hey, thanks for the great feedback...yu sold me! I was actaully leaning towards the 851 which is basically the same as the 47 Route 66 minus the wheels and pull handle. The reason I was leaning towards the 851...it lookes like, the access to the map pocket was a better. But, the 66 looks like it might be a bit more ridgid which is what I really liked. Thanks for the great info! Muddy