Do I use a circular saw or jig saw to cut pexiglass. What type of blade do I use?

If its thin plexi, score it with an utility knife numerous times and with a straight edge snap it like glass. This also works for Lucit and Lexan.
The thick stuff (1/4" or more) you can use a circular saw w/ teeth 60-100 or a jig saw w/ 10 minimum teeth.
The problem with cutting thin material w/ these tools are they either break when using the circular saw or the stuff melts and sticks to the blade when using a jig saw.
I would try the score and snap method first then resort to the saws if that doesn't work for you.

Chose between saws the same as you would if you were cutting wood. For the long staight sections use the circular. Any curves, use the jig.

Use the cheapest blade you can find because you'll have to toss it.