For a time last year Green Dot was removed from the Alliant Reloading Guide for 28 gauge. An article appeared in the Skeet Shooting Review about the same time citing, I beleive, excessive pressure which was damaging the chambers of some tube inserts. Recently Green Dot appeared back in the Guide again but pressures are usually the highest of any of the powders. If I were shooting tubes I think I'd select another powder; lots of good ones.
I use Longshot. I had used Universal clays till they came out w/ the new AA hull, and there was not a reload available in the chart w/ Universal. Now there is. I had my best luck w/ consistancy and patterns w/ Universal. The longshot so far has been fine, but if your shooting 20 and 28, why have 2 powders? Universal is a great 20 ga. powder too.