If really the main purpose will be regularly taking clients, you will find they will be absolutely crammed into an e90. Even the e60 is not a ton of room but if anyone will be riding in the back it is almost rude to cram them back there. Plus the m5 is much more of an executive car vs. a boy racer car (how others percieve the car which may be important if you are someone needing to drive around clients)
In terms of performance, if you are willing to get high-flow cats and a tune, you will be faster than an M5 in every situation that matters, meaning anything up until 120mph the m3 tuned/catless will be as fast.
As someone said however, the V10 sound IS simply amazing enough to make up for all of the downsides and worth it alone for this engine and sound.
For this case I think the m5 is clearly the right choice