Don’t quote me but I think coopedragon told me once the 6.5 was better comparing uvb output from different bulbs and to check a bulb for output and see how much decay the bulb has had. That way you can keep using the bulb until it’s stops producing useful uvb. A lot of the high quality uvb last much longer than a year.
They both do slightly different things, but either will work. I prefer to have the 6.5 to measure UVI which makes comparison to natural sunlight more straightforward. It will also measure the decay of a bulb over time. I like to have a UVI of around 3-6 in the basking area with a gradient down to <1 in other parts of the enclosure to provide plenty of options. With a T5 12% w/reflector you should hit that range at around 12-14'' or so.