Most recommend the 6.5r.
It too will show you your bulb degrading over time, but it has the benefit of giving you the UVI level, which is what every lamp manufacturer and expert is recommending now. That way you can adjust your lamp distance/basking distance as the lamp starts to degrade.
Say you want your basking area to have a UVI of 4 so that's how you set up your lamp. Well in 5 months maybe it's only reading a 3.5 now. With the 6.5 you can just lower the lamp (or raise the basking site) until you achieve a 4 again. Of course you can only do this within reason, but it's still nice to have the option.
With the flux reading of the 6.2, you are not able to do that as different bulbs give different UVI levels at different Flux levels. This is partially why lamp manufactures give recommendations based on UVI level, not flux.
The 6.2 will also allow you to see your bulb degrading, but it will not give you the standard measurement that manufacturers are using now. Both will work, but IMO the 6.5r is more useful for hobbyists unless you have a specific reason for wanting UVflux reading. If you are into that kinda stuff, I'd recommend just buying both lol.
-Brandon