Are Promo-DJ Copy stamped vinyl better?

Promos tended to be---but weren't always---early in the run of pressings. So better quality is a possibility.

Stamped ones are just probably earlier in the pressing run, and have no extra value. If they have a "Quiex II" sticker then they are rather special, as they are pressed on special, better Vinyl. You can tell if it is really still a Quiex II LP in the sleeve by holding the LP up to the light: Quiex II vinyl is always slightly transparent. I have unfortunately seen some covers marked Quiex II with ordinary vinyl inside.. So you actually have to check the vinyl. Quiex II is worth a premium over the regular LP.
The other great promos are where the LP label has a special label made just for promo copies. These are usually older LPs.. and collectors rate these as also worth more than a regular pressing. The final super special promo is when the LP is a different color, only for that promo. Those are rarer still. And finally, special records never released except as promos.. the record price guides often list those as being worth thousands...

No better then any other 'early' pressing.

Yes much better. Most of my collection are all promo pressed. Of course you do run into the risk of unplayable records or even unbalanced ones due to high frequency of usage. But still I think the risk is worth taking for promo copies.