I can say we have lost clients in the past due to mailbox sizes, as we can't keep up with the pricing MS can offer. We've also lost clients because oddly enough we seem to routinely get spam blocked by Microsoft servers (which is funny because they are the only ones that ever block our mail server). Privacy / data residency is also a hit an miss, as many clients have switch to Microsoft on-prem.
2 items that have worked for us, 1) Support. if you get locked out of your account, good luck with MS, were at least we can get you back in, and 2) more recently we've started to focus heavily on the mailing list feature of SM, even integrating it into our CMS. We've found it's MUCH more cost effective than some mainstream brands, although we do have issues with LARGE volume sends with our current systems.
We are hoping the new Market Place will bring in some new fresh plugin's to SM once the open source community really gets involved, but so far we are a bit under-whelmed...
Hopefully others can share some selling points that have worked for them.