I shouldn't really comment on the "who woulda been bigger" debate cuz I am incredibly biased since Aaliyah is one of my favorite performers of all time (if u dont believe it, click the link in my sig.) In what market? Movies or music, they both ventured into the film arena and I think Aaliyah coulda def went far in that area of entertainment. Beyonce? Not so much.
Musically speaking, I liked the direction Liyah was goin in artistically. I woulda have LOVED to see that collabo with Trent Reznor come to be, but alas.... I read a few comments on Aaliyah's vocal ability. If u copped the AALIYAH album, then u know she stepped her vocal game up. Creatively, the last album was refreshing, and I think that woulda def continued. The real question is, would the "popular" hip-hop/ r&b scene been able to appreciate Aaliyah's slightly-outta-the-norm vision as much as Beyonce's mainstream appeal??