i have an s5, although i considered the m3 and cayman s before purchasing.
if you want handling and performance, m3 hands down. also, value-wise it is difficult to beat the m3. i picked the s5 because, at the time, it was more unique. 335i/m3's were everywhere in los angeles--to bmw's credit for making such great cars. i liked that the s5 looked and sounded great with decent performance to go along with it. i don't drive fast often (rarely go above 70-75) and because my driving is mostly in the city, cornering isn't as important. i don't regret my purchase but i'd probably adopt different driving habits and opinion if i had an m3.
