So much for Sonic racing outside of a car à la Sonic R (well I guess it could still be a togglable option). But I really loved All-Stars/Transformed so if it's anywhere close to those games then I'm down. Shame it's likely true that the cast is limited to only Sonic characters, though.
Of the two modern ones, and as someone who has played every Mario kart, I think they compare very favorably. The first is a bit more of a standard kart racer but the tracks, handling, and items were wonderful. The second departs a bit from standard karts by being a bit more (just a bit) technically inclined; I had to fight more to overcome the AI but in a way that felt very natural and surmountable. And again, the various elements from track design to variety were exemplary.
I'd honestly give the nod to the Sonic kart games over most Mario karts, with MK8 maybe tying them.