Regarding the 6500 2.5-16x40, it tends to run out of light on higher power at twilight. The solution is to crank the power down a bit and then it's OK. The eyebox can get a bit tight at max power.
I don't like 50mm objectives in general, but with the 2.5-16, 50mm may be preferable to 40mm. I haven't compared the two, just thinking 50mm may perform better in dim light.
DF
