You know, after watching the Monday Night Wars series on the WWE Network (for $9.99!), I've come to alter my opinion on this subject. I'd always figured Hogan had had the greater impact.
Now, I believe Hogan came at the beginning of the increased exposure pro wrestling managed to garner for itself. He was a cartoon character who made people aware of wrestling even if they had never watched a match in their lives. His impact was enormous....and unique.
But the Monday Night War series showed how Austin came along when WWE found itself in a period of desperate trouble, struggling for survival in a time that WCW was looking specifically to destroy them. This is not a condition Hogan had to overcome in his time; no one significant was "after" WWE; on the contrary, they were in a position of expanding from a regional promotion to a national one. Their "opposition" amounted to a group of small promotions who wanted to stop Vince McMahon's grand plans but were helpless to do so. WCW, however, with Ted Turner (and eventually Time-Warner) money behind it, had no such restraints and had brought WWE almost to the point of extinction.
This is what Stone Cold and his buddies had to overcome. Yes, the contributions of many performers and back-office people brought WWE back to its former prominence, but the Network TV series indicated how massive Austin's impact was. He was the most popular wrestler in the world and, like Hogan, folks who knew nothing about pro wrestler soon learned who he was. Of all the superior performers in WWE, Austin took the company on his back and forced us to think about him. Everything was built around him, and he proved capable of carrying it. Through the years, I'd forgotten how enormous was Austin's impact and influence; this TV production brought it all back.
In all, I'd say Austin was the bigger star because of what he had to help overcome.....as opposed to the clear, unfettered shot at the brass ring enjoyed by Hulk Hogan.