Actually, it is true that fixed scopes (all else being equal) are brighter than variables. The reason for this is because fixed scopes have fewer lenses. You know how certain companies say that they have 91%, 95% etc. light transmission? Well, they mean "per lens." As such, the more lenses there are, the less light transmission there is.
As for the weight, I think that there are some compact scopes out there that are just as light, if not lighter, than fixed scopes.
Still, I don't want you to think that I am necessarily forcing you to get a variable scope. Granted, at least to me, I don't think that a couple of ounces will make a big difference, even at the end of a long day in the mountains, but I respect your decision either way.
Accordingly, as for your specific question - fixed 4x or 6x, if I had to pick one of the two, then it would be the 6x, and the Leupold is a good bet.
Zachary