My M3 Vert is perfectly confortable on long trips and the adjustability of the suspension/power/steering and seating makes a big difference. Top down wind noise/buffeting is minimal which makes long top down drives great. Drove the entire Grossglockner in Apr '11 with the top down, beautiful! My brother has an S-5 so the comparison is easy. The S-5 interior is more "elegant" and the cabin is a bit bigger, especially the back seats, but I really don't find it more comfortable than my M3.
The only downside I have is tire noise. The M3 comes with three possible stock tires, Continentals, Michelin and Pirelli. I have the Pirelli P-Zeros and have a very loud rumble from the tires that started to get noticable about 15k miles. At 20k I'm about to replace the tires just because of the noise and would recommend avoiding the P-Zero's which happen to be one of the OEM tire options for the S5. If you're looking at a used M3 then I'd definently plan on replace the P-Zeros (if that's what it has) with Michelin Pilot Super Sports.
YESTERDAY: '08 335i Coupe, Space Gray, Coral Red, Gray Poplar, 6-MT, ED 10 May '08.
TODAY: '11 M3 Convertible, Mineral White, Fox Red, Extended Leather, Carbon Leather, 6-MT, 19" Style 220M Wheels, EDC, EPS, ZP2, ZCV, ZCW. ED 29 Apr '11. PCD Re-delivery July 27.
Last edited by Mace14; 01-10-2013 at 01:25 PM.