Green bus, far more comfortable and safer than a van. Book at least a week in advance for a VIP seat (they do sell out). It is a long day, if you're not up for ~12 hours break the trip and overnight in Mae Sai. Cheap hotels, reasonable sort of market and food. Tachilek itself is only slightly more favorable than cancer but if you want to do something different visit the casino (about a 15 min ride) it makes for a good drinking story :)