Quick comparison
The Ferrari 290 MM appeared in 1956, the Ferrari 290 S in 1957 – one year later. Both have a V12 engine, mounted at the front. The more powerful one is the Ferrari 290 S: 330 PS versus 320 PS. The rarer one is the Ferrari 290 S: 2 units versus 4.
All values come from the data sheets of the two models (sources listed there). The better value is highlighted in green wherever both data sheets give a comparable figure. Photos – Ferrari 290 MM: Tino Rossini from Toronto, Canada, CC BY 2.0. Licenses and originals: Sources & image credits.