I was just wondering if rogue trader gets best-craftmanship storm trooper carapace ..or common-craftmanship?

..it's not specified in the book.

Common Quality. Which means that a smart player will take the enforcer light every time as it provides the same AP with less weight.

If the stormtrooper option was best quality, then no-one would take the enforcer light as the stormtrooper gives 1 extra AP for the same weight.

Personally I'd suggest offering the player a choice between best quality enforcer light (the lighter armour) and good quality stormtrooper (provides more AP against the first hit) so that you don't have a case where one choice is equal or better in all areas.

Yes, it's 'just' common quality, but it won't be that difficult to aquiry some better quality armor for your charakter. Remember, you get a +30 on your profit-roll, if you only want a single item, which means the chances for getting some best quality stormtrooper armor arn't that bad... if your GM allows for it, of course.

On the flip side, if your GM has you making Upkeep Tests with regard to your armor, you may want to go with the common quality Storm Trooper Carapace instead, especially if your Profit Factor isn't that high. The modifiers to the upkeep test come to a net amount of -10 for Best Craftsmanship Enforcer Light Carapace (Rare Availability -10, Best Craftsmanship -30, Negligible Scale +30); for Common Quality Stormtrooper Carapace, it's +10 (Very Rare Availability -20, Common Craftsmanship +0, Negligible Scale +30). As always, use your best judgment on the matter.

-Kirov