[Soliditude]

Biggest difference are the engines N54 for the IS and N55 for the regular. The power on a regular coupe can be matched, practically, if you get the PPK upgrade. That'll give you all the bonus HP/tourque. As from people whining about single vs. twin turbo, the N55 has a sophisticated, twin-scroll single turbo, not just a single turbo. It won't be as good for tuning options, but it's more reliable and more efficient. That's really the main difference other than the cosmetics and the DCT transmission in the IS.

The 335is is more rare and more exclusive, as people mentioned, and if you're planning to get upgrades for a regular 335i, then might as well just go with the 335iS right off the bat and solve all the business and stupid dealerships later. I would personally love to have an iS if I could.