I've toured in Thailand, Laos, & Malaysia. Dec-Feb is the ideal time to tour in northern Thailand and northern Laos, as those 3 months are sufficiently north of the equator that they have significantly more comfortable temperatures than during the rest of the year. It also coincides with the dry season. I did a wonderful loop heading north from Chiang Mai along the borders of both Burma & Laos, going as far east as Nan before heading back westward toward Chiang Mai. I also rode through the northern half of Laos from Vientiane toward the Chinese border, then westward back toward Thailand. Both of these rides were superb with great scenery and good roads. Touring in Thailand is very straighforward, as easy or easier than touring in developed countries. Laos is much more basic than Thailand. A few places I rode through in Laos were off the electrical grid. Accommodations were sometimes primitive outside of the larger towns in Laos. I agree with the advice above about leaving camping gear at home. Don't bother with a stove, either.
There are trains in Thailand which can be very useful. I took a night train from Bangkok to near Vientiane, and it was very easy to take a bike. There are also trains between Bangkok and Chiang Mai, as well as a few other routes in the country.
Overall, I thought touring in Thailand was better than in Malaysia. It's harder to find quiet roads in Malaysia. Most of my biking was on the east coast of peninsular Malaysia, from Singapore up to Kota Baru near the Thai border. I was also on Penang, a wonderful island off the NW coast of Malaysia. Penang, and especially its main city Georgetown, are absolutely superb to visit.