Hotel: Four Season Resort Lanai [beach]
Room Booked: Garden View
Early Check-in: n/a
Late Checkout:Proactively offered and used for 2 pm transfer to Sensei
Upgrade: Partial Ocean View
Credit: Supposed to be for Spa services, but they told me at check-in that it can be a general resort credit. It was not credited at such automatically, but they corrected and applied to some F&B at check out
Breakfast: $48 per person per day at One Forty. Portions were pretty filling, so we split 1 hot breakfast plate along with some additional pastries, fruit and sides.

Hotel: Sensei Lanai, A Four Seasons Resort
Room Booked: 1 King Koele
Early Check-in: Yes, at 2:30 pm
Late Checkout:Offered but not needed
Upgrade: none
Credit: $100 F&B, used at Nobu
Breakfast: $48 per person per day at Nobu. Portions are lighter and smaller than at One Forty at the beach resort, which I actually liked. However, the iced coffees were a lot more, $9 at Nobu Sensei vs. $5 at One Forty at beach resort.

Comments about both resorts: I think both resorts on Lana'i are "special occasion" resort because we kept on getting asked if we were celebrating anything. [We came to basically check off all the FS resorts in Hawaii]. Sensei is explicitly adults-only, but I only saw 1 kid at the beach resort my entire stay. September is considered low season, and was told that the beach resort did have more kids a month ago. Based on conversations I overheard, guests are allowed and get shuttled to eat at the other's restaurants, but you can't wander around enjoy the amenities. One Sensei guest was denied a pool chair, but was welcome to eat at Malibu Farms. Maybe they do allow Sensei guest to use the beach chairs since the beach chairs are public. It seemed like all the beach resort guests were couples (and 1 kid), but Sensei also had friends or friend groups visiting.
Overall, I would more likely go back to Sensei over beach resort because it's unique among the FS Hawaiian resorts. It's all down to personal preference, but the beach resort is kind of a more secluded but also scaled down version of their resorts on the other islands. The pool facilities, while never crowded, very tiny in comparison. While both beach and Sensei have the best onsite restaurants with their Nobus, I thought the menu at Sensei Nobu was more unique as the beach Nobu had almost the exact same menu as the Nobu restaurant/hotel in my hometown. Also due to the distance from town, it's more of a schlep to seek out non-resort food compared to at Sensei, where you can even walk it if you want. At the beach resort, we did the observatory experience which was really cool. At Sensei, we rented a Jeep for a day to go explore the island (since all roads lead into town, you do save time started out from Sensei). You should be able to call one resort to book activities at the other.
Finally, the Lahaina fires have also greatly affect Lana'i, as they've lost almost all their day-trippers coming by ferry. The Lana'i ferry still runs, but out of the farther Maalaea Harbor and not daily. The restaurants in town don't seem to get much lunch business, and the Cat Sanctuary went from getting 100 people/day to single digits.