In the world of fluorescent minerals, more UV power is better. If the very powerful lamps are out of your price range, you will want to use a smaller case. The
9 watt units will work with a small display case — up to a 1 foot cube or so — but you will want a unit with a fan if the lamp fixture will be on for long periods of time.
The 9 watt units are OK at about 1/4 to 1/3 of a meter high with a spread of about 1/3 of a meter.
A 36 watt SW (UV C) lamp fixture will light a standard case that is 2' tall x 2' deep x 4' wide, BUT the minerals at both end of the case will not be lit well so you would need specimens that fluoresce brightly at the ends. According to Philps a 36 watt tube/bulb emits 12 watts of UV C
A 60 watt SW lamp fixture will light a standard (2'x2'x4') case fairly well and will reach out to the ends, but it will not be bright if you raise the lamp much higher than a standard 2' tall case. A 60 watt bulb emits 18 watts of UV C.
A 95 watt SW will do a good job in a 5 foot tall case if the specimens at the bottom fluoresce well. A 95 watt bulb emits 32 watts of UV C. We can also make a unit with a 95 watt SW and a 65 watt (or a 60 watt) LW bulb.
A 120 watt SW unit has two 60 watt bulbs so it emits 36 watts of UV — a bit more than a 95 watt unit.
A 190 watt SW unit has two 95 watt bulbs so it emits 64 watts of UV C. The Franklin Mineral Museum in New Jersey recently revamped their 28' long display with 8 of the 190 watt SW units.
As for dual units, to my eye, a unit with 36 watt SW and 36 watt LW always seemed a little miss-matched on response in a group of mixed SW and LW fluorescent minerals. The LW minerals seem brighter to me than the SW minerals - yes, the LW minerals. To my eye, the 60 watt SW/ 36 watt LW combination gives a very good blend of SW and LW. I think the answer lies with the filter glass. The best SW filter glass available is made by Hoya of Japan. However, transmitting SW UV is not easy. Hoya glass will transmit about 64% of the SW UV when the glass is new.
As the Hoya glass is exposed to UV C radiation for long periods of time, the UV transmission decreases until it eventually needs to be replaced. The LW glass on the other hand passes about 85% of the LW UV and never wears out. Now let's look at some realistic numbers. According to Philips, a 36 watt LW tube emits a total of 9 watts of UV A and a 36 watt SW tube emits a total of 12 watts of UV C. Not all of the total UV energy will be directed out through the filter glass, but we can use the numbers that Philips provides for a relative comparison value because the two lamp fixtures are of the same design. If you crunch the numbers, 64% of 12 watts (for the UV C tube and filter combination) is almost exactly the same as 80% of 9 watts (for the UV A tube and filter combination). They both end up with about 7 watts of UV available.
Reference: Gardner, Bill. “Frequently Asked Questions.” Frequently Asked Questions | Way Too Cool LLC, https://www.fluorescents.com/faq.html#display.