Admittedly I don't know much about Nikon, so good to know someone out there has a reasonably priced superzoon for FF. Since you mention that you actually carried the 28-300L at an amusement park all day, what are your impressions of the lens (from a father's viewpoint)? I'm curious because I'd use it for pretty much the same thing. I'm guess it doesn't do so well indoors, but that's not what it's made for.
Quite useable, IMO. Definitely need a Blackrapid strap or equivalent to carry it, though. Across the range, the IQ is basically equivalent to the 24-105L - not stellar, but very good and fit for purpose.
Depending on the body, it can do ok indoors, but really only on the wide end and you have to push up the ISO. The first shot below, from a trip at the beginning of this past summer, is with the 28-300L on the 5DII (35mm, 1/80 s, f/3.5, ISO 6400). The second, taken on another trip later this past summer, is with the 35L on the 1D X (1/160 s, f/2.5, ISO 6400).
Actually, I didn't plan on this being a formal comparison, but given that it was the same location, same focal length, same approximate framing, and same subjects (although from a different camera angle and separated by about 3 months in time), it actually makes a pretty good real-world comparison between the two setups (5DII+28-300L vs. 1D X+35L). The composite below are 100% crops from the two images (1D X+35L on the right). I think there's a clear winner...