I had a look at these when thinking about a Z4 but with the roof down there didn't seem to be a lot of boot space, or at least access is very poor! My main thought is if I need to travel with luggage I don't want to have to have the roof up!
Also, the 35i is the 3.0 with a turbo, so it depends if you like the low down surge of a turbocharged car or not or prefer to rev to get your power?! I haven't driven either but I'd imagine the 35 gives a lot more 'shove' at the lower end as a result...