First of all: We have a vacuum shop in my city that actually specializes in Sebo.
Anyway, now onto Kirby vs Sanitiatire:
You know Im a Kirby buff, but thiis is a pretty fair fight.
What it comes down to is this:
Sanitaire is made for COMMERCIAL cleaning, this means it probably is going to work best on commercial of berber carpet.
Kirby is madee for domestic use, (Although I do know places that use Kirbys as industrial vacuums, to clean corridors, offices, etc), this means it will do better on low-high shag, and berber and commercial carpet, and it also is more versatile, but I guess not what this topic is about.
All in all, I would have to go with Kirby, being more versatile on different carpets.