For the cost of G Suite it's a no brainer.
I don't know. I mean, spam filter is all solid, but like I said if native mapi is a must - ie Users insist on using Outlook and all it's glorious functions (imho installing GSSMO seems not elegant solution)
Also pricing wise. $5/user/month is surely cheap, but only 30gb storage.
Rackspace hosted exchange is $7.99 for 100gb mailboxes/user. Addon for email archive is only $3 extra.
G Suite could be even cheaper with annual plans, but I'm not sure they are still available.
So pricing is not far off at all.
Never has an issue with them (RS) and their
support is actually excellent. They helped with free migration, fully handled by them from Mailstreet whom decided to stop providing hosted exchange.
Not trying to change your mind about this particular customer of yours. Just sharing my experience.
I think G suit is much more than just email, but how many of its functions your clients will actually use?