I don't think the loop you choose will make a huge difference. All of the islands have their own unique charm and wildlife. I think that the water temperature will likely be warmer in more of the northern ports, so if plan to snorkel, that could make a difference. The water temperature down there is freezing by the way in many spots, even in a wetsuit. Also, you can check on if the southern loop crosses the equator and if that matters to you.
The only way to get to the galapagos is to fly into quito or guayquil. If you don't intend to sightsee in ecuador, more than likely your flight will land the evening before and then you will fly into galapagos the next day. I know that Celebrity does a chartered flight for that part, I don't know if you'll take the chartered flight if you don't book the package? (we did the galapgos, but not with celebrity)
We spent a day in quito on each side, mostly because of the covid restrictions at that time. It's certainly worth spending a day in. For ease of arrangements, I would book the 10 day package. There are other fees and steps required to fly into the galapagos on your own.