Hi,
Beaten by numerous others as I hammer at my netbook keyboard, but I may as well add my two-pennyworth.
Yes that "Upgrade" is only to charge rechargeable batteries, not to give Solar data (which needs a different base station to decode/display the additional data). Definitely not good value as a pair of Lithium AA cells may give similar lifetime (as the rechargeable Alkalines) for just a few Pounds.
I'm almost certain that both 3080/81 have loggers, but the 3081 does NOT have the automatic radio "atomic clock" setting.
If I was buying now, I might go for the 308x series, but I do have some doubts about the usefulness of the measurements. The illuminance is (or should be) weighted to the human eye (not for example to energy-saving PV cells) and the UV index seems to be such a critical measurement that I doubt if FO get it correct.
Cheers, Alan.