Having migrated from Dovecot/Postfix/Amavis to Exchange, I can tell you: Exchange requires more resources, more administration, and more money. You will gain benefits from Exchange provided that you have an existing Windows AD domain that is "sane" and ready for it. The exchange box is beefy: 4 cores, 32Gbyte RAM, and 1Tb of storage. You'll also need an admin that understands it.
Google's services are certainly seductive, but I'm still not sold on handing over control of a fundamental piece of infrastructure to an outside source. And what's with 99.9% uptime? I suppose that's ok for some use-cases, but if you're doing sales, or customer relations, being offline for about 8 hours could be difficult. Otherwise, it's easy to see how it would make sense to outsource this function.
Just for comparison:
The old Dovecot/Postfix/Amavis setup hosted 60+ users at its peak on 2 CPU cores, 6Gb of RAM, and about 250Gb of disk space. We had a lot of email, with some email boxes going back years. Volume was very light, about 4-6k messages a day, and about 750,000 non-spam messages delivered per year. Given the recorded stats from the server, it could have had the hardware maxed out to accommodate between 1,000 and 1,200 users. Other than adding and removing people from mail aliases, it required no intervention.