Bus and Boat between SR and Battambang

As #1 said, no need, they do pack 'em in like sardines during high season, but booking ahead won't give you an advantage.

I did this trip recently, and it was much less crowded from Battambang to SR. I think it's because many people go from Siem Reap to Battambang and then on to Poipet for Thailand.

You couldn't book it ahead if you wanted...

We'll be in Siem Reap mid or late November, after traveling from Hong Kong and through Vietnam.

What do you think of taking the boat to Battambang for a couple of days and then back to S.R. to hop a flight back in the direction of Hong Kong (like Shenszhen, or Macau)?

Or should we go from Battambang to Bangkok or Phnom Penh to grab a flight there? Should we skip Battambang?

I'm not sure Thailand should be part of this trip, for the couple of days we would need to just manage to get a flight out.

Obviously there are many more airlines & options from BKK. Up to that point most of our travel will be overland and to maximize our visiting time and not backtrack too much I think flying out will be better.

#5 - I would arrange your itinerary such that you only have to take the Battambang-Siem Reap boat once. After you've done it, you'll probably not want to do it again.