I'd put both on if there are places for two. Panaflo low speed ones are very quiet. Jab-tech.com and SVC.com and many others carry them . IDK how the Yate Loon 90mm ones are, but the 120s are generally pretty quiet. The panaflos with the letter "X" in the suffix of the model no. (e.g. FBA09A12L1BX - this is the one to which I'm pointing you) are the ones with the speed sensor - probably no need for the speed sensor on HDD fans, CPU and case exhaust and PSU are the ones I would want monitored - any of those go down and the heat she rises dramatically... No X, no speed sensor.
Some resellers will try to trick the unwary by supplying a 3-wire lead (panaflos have connectors on the fan and don't come with leads, the reseller either includes one or sells it separately) on fans that don't have the sensor - subtle, eh. Aren't you glad you asked here?.. 😉
.bh.