X-Pac was most definitely more over than Mankind at the time Mankind's main event push started. Mick was not getting strong reactions for a good 2 months in that position. Actually, Mick had real issues getting and staying over with WWE crowds. People tend to forget that going into HIAC, he has NO heat at all. After it, he didn't have much more, and started turning into a comedy joke character who suddenly was in the main event picture to keep Austin and Rock apart before WM.
The lowest buy rates for PPVs after Austin won the title in 1998 both involved Foley in the main event. Over The Edge (Austin vs Dude Love 2) did 500,000 less than WM, 100,000 than Unforgiven. Rock/Mankind at Rock Bottom fell in between OTE and Unforgiven. As a headliner, Mick wasn't doing shyt for ratings/buyrates in 1998. And after that, he was never the TRUE main event attraction, since his next two matches took place on the Rumble (with Austin and Vince in the rumble itself) and was semi-main to Austin/Vince in a cage. Then a triple threat with Austin/HHH that wasn't made a triple threat until a few days before the show.
He only had one PPV main event where he was truly the main event and star, and that was No Way Out 2000 (his retirement show), and that did better numbers than the Feb PPV the year before. But that was after a year of main events and being buddy buddy with The Rock, the biggest draw of the period. In 1998, he wasn't very over and was in fact struggling to keep his head above water.
However, X-Pac was never even attempted to be pushed in the main events, so there is really no way to quantify who was a bigger draw or anything. You can definitely HEAR the difference in their reactions in 1998, though.