Man, that’s a hard one!
Both were fast and nasty. Senna was the hero back in the days I started watching F1 (I’m Angolan and like in Brazil, “everybody” was rooting for Senna).
I was very young in 1992 (8 years old) but my man was Nigel Mansell, as a 8 years old his moustache could not be ignored. Years went on and by 1994 I was completely taken away by a young man with “weird” name: Schumacher (for a 10 years old Portuguese speaking boy, that was a different and lovely sound) and since then he took over the place of Mansell as my favourite driver, his name, car livery and then driving skills won me over.
Schumacher’s success and some regrettable moves from his side turned him into the main hating target in Formula 1 history but I was there until I got tired of his dynasty that manage to remind me how big a name Ferrari was (because before him Ferrari sucked for a kid who started watching F1 in early 1990s when it was all about Williams, McLaren and Benetton).
For me, it’s Schumi but I understand different opinion because the difference is not that big.