Personally, I never sell anything other than the Pro edition. The price isn't that much different, especially when you compare the cost of 3CX to other solutions. Everyone needs to remember that you must have the Pro edition to run the CRM plugins, or to implement Call Flow Designer applications, (and with SP3) use your own FQDN and SSL Cert, and SMTP server.
Since nobody has a crystal ball where they can be assured of the customers future needs, selling exclusively the Pro edition means you never need to have that unpleasant conversation where you have to explain to the customer "if you need that feature you have to pay more". Better to just handle that issue from the beginning.
Remember, in the overall cost of the project (hosting/server, phones, gateways, network upgrades, labor, SIP trunks, etc.) the difference in cost between Standard and Pro is almost nothing. Just sell the Pro edition and give your customers all the available features. If the customer sees the price difference as a deal breaker, then you probably don't want them for a customer anyway because you will be fighting over the bill every month.