I don't own a Safariland holster, but I do carry an XD45 in a Serpa. Most of the time I carry in leather, but as for the Serpa:
It is the only poly holster I like, can be worn for OC and CC (though it sticks out farther than leather in CC)
It is also the only paddle holster I like... I also have a Fobus and the Serpa on paddle is far superior in retention, comfort and stability. The "butt flop" I always experience on a paddle almost disappears with the Serpa. It is not as steady as being on a belt, but it's dang close.
Add to that the drop-leg, thigh mount platform from BLACKHAWK and my Serpa is the most versatile holster I own. Granted, this piece does not come with the Serpa like the belt and paddle mounts.
I really like the button where it is on the Serpa. I also trained to draw with my trigger finger parallel to the slide, extended, above the trigger... "at the ready" she called it... so the Serpa becomes very natural, very quickly. As to whether it presents a danger of AD during the draw, I would say that has little to do with the holster and everything to do with the finger.
Having it in a similar position to the thumb break on my leather units would be cool too though...