How a 435i gets to $75k without any options is... staggering, but I'd think one might be able to get a M3 Cab without some of the options I got for $75k.
If it matters, I'd vote for the M3.
The 435i convertible with useful options is quickly around $65,000, and then driver assistance and similar toys catapult it to 72k easily. That is why it is in here as "tricked out".
Having said that, you can easily configure a 435i and a e350 convertible to be the same price both with navigation system, and quite frankly I perceive the E-class as higher value, with nappa leather, ventilated seats, some air guidance gizmo (which might be useless but maybe not).
The absolute minimum for a 4-series convertible if you want neck warmer and good stereo is $56,000. At that price I might even consider leasing an Audi instead.
ETA: I think the confusion might be coming from me saying "no DCT, no AWD, no LSD". Those things are not available for the 435i convertible, optional or not. If they were, e.g. making a new 435is then it would be a different matter. But you only get to have the same automatic transmission as everybody else, there is no AWD option for the 6-cylinder with the convertible, and limited slip differential is aftermarket only. No stick shift either.
Now compare that to an Audi S5 convertible if you want those actual sport options. The BMW only gives you the nice engine and you might or might not like the suspension offerings. But the rest of the sports car isn't there.
Now, it isn't all bad. The c63 is the classic example of a car that is very expensive and only has a large engine to show for it. It doesn't have any of those options either except you can sometimes get a LSD. But the c63 has a very, very nice engine and it is quite reasonably priced if you account for the options. There isn't any of the 4-series convertible options packaging ripoff going on.