Welcome to the boards. My vote goes for 1/8". I'm not a molding fan, but if you use a locking strip style of molding, you'll need to make sure the thickness you use will allow it to be admitted into the groove in the molding. I can't think of any advantage to 1/4" at all unless you want to do a deep etching in ot.
Thank U very much! Would u please tell me where I can buy those acrylic sheet and window molding with locking strip? I'm at US.