If you have not already visited Battambang, it's worth at least a day--I spent 2 days there and could have used another. It was an unexpected highlight of my trip to Cambodia about 18 months ago. You won't see many tourists around and the locals are great. I used Seng Hout hotel, very nice ensuite with a/c for about US$15, good location. I did the bus from PP and either direction, it's about 5 hours (with pitstop enroute) over a decent road.

Alternative route: Chanthaburi to border to Pailin to Battambang. I think buses start running from Bangkok Ekamai to Chanthaburi about 5 am. Minibus (?) Chantha to border. I'm pretty sure this border does Visa on Arrival similar to AP/Poipet border, but it will be much less crowded. Road Pailin to Battambang was legendarily bad but in 2009 they were upgrading a good stretch of it out of Battambang...maybe Tales of Asia website will have an update on current conditions and if this highway has been finished. Watch out for mosquito bites in this part of border area--very notorious for ultra-bad resistant malaria. Look at Canby website which keeps this stuff pretty up-to-date. There's apparently another crossing at Ban Laem between Chantha and Battambang.
http://www.canbypublications.com/cambodia/overland.htm

Not going to be any faster getting to PP but might be an interesting journey if you've already done the Aranya Prathet thing before. Given your arrival time, you've really got a dead zone getting to anywhere for the next 5+ hours unless you want to pay for taxi, get to the border, and wait until 7 am for it to open.