Depends on what you are into...
BKK will be filled with the "white party" crowd, younger white ravers that think they are gods gift to mankind and their admirers, along with Chinese/Taiwan panda types doing their conga trains.
Clubs will be mobbed and dangerously overcrowded, long queues will ensue. No taxi will want to take you anywhere when you are dripping wet.
Pattaya will be filled with the Songkran "terrorists" that some of our members seem to be so scared of. Unlike Bkk, the water play is probably going to go from April 12th straight through April 20th, based on recent years, nightfall is no longer the guaranteed ending time (especially in Soi 6/7/Buakow/Beach Road and Jomtien Complex)....It will be mayhem, but at least you can get transported on a Songtaew.
Whichever location, as long as you are willing to go with the flow, smile and be happy, you will have a ball! Just booked my flights and cant wait for it!