I don't find a difference in smoothness, but I get much better fuel consumption on 97 Octane fuel as against 95 Octane. 97 is about 5 to 8 pence a litre dearer than 95 for me. They have 98 in continental Europe and it's pretty much the same price as 95 (unless I'm being dumb with the currency change, not unknown).
My manual warns me to go gently if using anything less than 95 and not to use less than 91.
1999 SL 500
1993 320 TE
1986 500 SEC