I think they are as mad as each other, driving cars around on a lake (frozen or not )
As F1 drivers it's really tricky - I think both of them are/were top of a second tier of drivers, just below the greats of the sport. Nico is definitely more consistent. Keke could be unbelievably fast on his day but I don't think Nico has been given enough credit for how he has done against Schumacher although I guess it is easy to blame Schumi's age for that. I don't think Nico could have done any more in his career to date with what he has had at his disposal?
I would like to see Nico against Vettel, Alonso or Hamilton in the same equipment at some point. Prost blew Keke away when they lined up at McLaren but until Nico can prove himself against one of the top tier drivers, Keke's race wins and world championship (even though he only won once in 1982) would tip it towards him.
Jez's summation sounds pretty reasonable to me. On his day Keke was as good as they come, but after 1982 his performances were patchy. His first win, in the rain at Silverstone in 1978, was against some seriously good competition (Lotus in their pomp, with Mario and Ronny, Niki in a Brabham, Hunt in a McLaren). Comparisons with Prost in 1986 are a little unkind as the McLaren that year was definitely not to Keke's liking, and on top of that his head was in a bad place that year. The fact that he literally walked away from his stricken McLaren at the Australian GP and never sat in an F1 car again shows that although in later years he claimed he had retired too soon, at the time he was more than ready to give it up. Only the year before he had come out fairly evenly against Nigel Mansell, who I don't think anyone would describe as a slow driver. Nico still has to do something special, I think. Once in a while he'll show flashes of inspiration, but as they years roll by he's coming over more like a successor to Nicko Heidfeld than his dad.