Cindy,
Is there a reason you wouldn't suggest using the D365 Field Service app?
It does everything ExpandIT does and more. And has the advantage of being a 1st party MS native app. I will be the first to say that if you had posed your question 3 release waves back, ExpandIT would have been my choice as well. But MS has massively improved it. Heck, it FINALLY has FLOC's! And you get the native Bing maps integration, obviously. Of course, if the client wants to go all in MS also offers RSO (Route Scheduling Optimization), which ExpandIT can't compete with at all as far as I know. And the newest version of the mobile application for the trucks, that is built in Power Platform, is wayyy better than the older Xamarin app.
Of course, if they go with the Native D365 FS, there is also the opportunity to introduce D365 Sales, CS, and even Marketing (though I do find MS' pricing for Marketing to be tough to swallow unless the org can and will really, really leverage everything it provides). Plus Customer Voice for immediate after-service surveys and such can also be added in.