As a costumer, this is what you need to know between Styrene and ABS:
1. Styrene costs less (generally 30-50% less)
2. Styrene is much more brittle (especially in thin plastics)
3. Styrene is easier to Vac Form because it does not 'Boil' (orange peel effect in ABS that can be controlled by drying the plastic prior to forming). This has lead some people to think that Styrene gets better detail. Wrong! They're super-heating the plastic, which allows it to stretch more and become thin. The thickness of the plastic and the amount of vacuum is what determines the detail when Vacuum Forming.
4. Styrene is a pain to cut - see #2
5. Styrene only comes in Matte. 'Gloss' styrene has either been buffed or coated with another material (automotive clear coat is a wonderful UV inhibitor)
6. Styrene takes paint easier. Not sure if this still rings true, since the introduction of paints with elastisomers (Krylon fusion).