Either would be fine as you have plenty of time even if the 20:35 TG flight arriving a little late. It really depends on where you want to spend those extra hours.

I'd personally go for an early dinner by the beach in Phuket and then leave for the airport at 18:30. You have plenty of time to check in for the KAL without any rush.

With the earlier TG arrival option at 17:50, I'd probably do an early check in (assuming that KAL are open for check in) and then take the Airport Line into town for some dinner for a couple of hours. Then back to the airport on the ARL by 22:30 at the latest. Immigration won't be too bad at that time especially the east zone....west zone is usually busier from around 23:00 onwards......