I'm surprised the tech gave you grief. The few times I've had dealings with the HVAC people and from what I've read online, they tend to stick with the cheap fiberglass filters and justify that by saying the filter is just there to keep large debris out of the blower. If you want air purification, buy a separate purifier/filter.
That having been said, when I've upgraded, I've used something in the MERV 7-8 range.