(I'm not sure whether this question belongs in the Kindle subforum or the Calibre subforum, so I guessed.

If it's in the wrong place, I trust that one of the moderators will move it).
I've already searched MobileRead and although this is probably a basic question, I couldn't find that it has been answered.
I'm considering the purchase of a Kindle Paperwhite 3 for my wife. I use Calibre, with Apprentice Alf's plugin for the few "commercial" eBooks I purchase. However, as a longtime Nook and Project Gutenberg (PG) user, almost all of my library consists of PG .epubs: ca. 300 titles.
Here's my question: What would be the best strategy for sideloading PG titles to the Paperwhite 3? Based on my initial research, I plan to sideload (via USB) from Calibre to the Paperwhite -- the "Send to Kindle" feature doesn't appeal to me at this time.
Since the PG titles are all available in .mobi format, how about importing them into Calibre and then sideloading them as .mobi files? Or would this be wasteful of space because the .epubs already exist in Calibre's library (and it can convert the files)? Would Calibre complain about "duplicate" titles?
Would it be better to allow Calibre to do an "on-the-fly" conversion to AZW3 and then sideload to the Paperwhite?
Just two other tidbits:
- I don't plan on converting all of the PG titles because my wife's reading tastes are different than mine. I'll only sideload a limited subset of works she's interested in.
- I don't plan on asking Calibre to do a batch / mass conversion. I would take the time to convert the selected titles individually.
I'd appreciate hearing your opinions and any advice you might offer. Thanks in advance for your time and expertise.