In context of the time Punk's promo had more impact on wrestling than Austin's did hands down. Austin's promo was your typical promo directed at a single wrestler to out-talk that wrestler. Was it funny? Yes. Was it different? Sure Did it change the direction of the company and the way story lines were written? Not really. Punk's promo however delivered on all fronts in this PG era of lame programming. Just giving Punk the chance to do the promo was a fresh escape from the "All hail John Cena" attitude the company was going which also took the spotlight off their golden boy and gave an unexpected superstar a chance to show what he could do. Punk's promo was also more real in spirit because it had truth behind it (whether or not it was actually scripted). Not to say SCSA did't have attitude, it did, however devoid of emotion as Punk produced. Also look at what happened in WWE after Punk's promo. Cena lost the title IMO the best match he has competed in, unique story lines emerge with the belt leaving the company (which should have lasted longer IMO) and cha few in the front office (Mr. MacMahn stepping down as COO), Punks epic title run, etc. Punk created a hell of a lot of more change in the business during the time of his promo then SCSO did. Oh yeah Punk sold a few more t-shirts too.
I have to go with Punk.
SCSA was on his way to the top with or without his 3:16 promo. People loved him and his attitude irregardless. The 3:16 comment was made to made to Jake Roberts and his new found Christianity and turned into a nice hook and T-shirt.
Punk's shoot, on the other hand, was a shot in the arm to the whole WWE. Things had become stagnant and to see someone break down that imaginary wall between being a "wrestler" and being a "person" with all the real emotion behind it - it was something special to say the least and catapulted him to the top of the World for being the one to speak out and have a real voice and be true to himself (it naturally helped that he has solid skills in the ring as well).