Acrylic. There is no comparison.
Acrylic beats glass in pretty much every way with the exception of scratches. Since you will not really see the sump, scratches are not really a concern. If you are getting a custome built sump, price might be a factor, but you can do so many things with acrylic that you cannot do with glass (at least easily). With acrylic you can drill holes for probes, equipment, to hold filter socks etc. If you are making a sump out of an old tank look for someone who has scratches the heck out of their tank and you can usually pick it up for a song and dance.