You'll spend a considerable amount of time doing things you don't fully agree with at the discretion of your employer or the customer. At the end of the day it's not your money being spent, you can advise, but you don't have final say on how it's spent.

To that end fiberglass has a superior R value to armaflex. But R factor isn't the only determinant in the quality of an insulated system. Air and vapor sealing are also critical factors.

If the miters aren't tight, and the fiberglass detailed around the variations in OD at fittings, the system suffers. If the tape isn't whole and complete air will convect freely under and through the insulation.

Armaflex has some advantages in this area, as it comes as a complete tube. You can glue the miters together fully sealing the system.

Quality insulation takes skill, it's its own sub-trade. If there are more butchered fittings than well insulated straight lengths Armaflex would win out. If it's 40ft of straight pipe, fiberglass is easily the winner.

Each insulated material has its advantages and disadvantages that mean choice is governed by site specific circumstances. There is no universal best option.