IMO, the Fort Cast Members are notoriously tight lipped about giving out your assigned site number until it is time. For example, if you check in and they are full (common around holidays for example) they will not tell you what site you are pre-assigned to. They will make you wait until the prior guest leaves, the site is cleaned and prepped, and thus ready for assignment. Then they will let you know.
If you tried to extend, I guess they were full in the Premium category for one of the days following your scheduled visit. Requesting a loop is a very personal decision. Do you want it for its location in the Fort (close to ....) or the amount of shade.... or close to Fort amenities like the dog park... or do you want a small loop so no matter where you are, it's not far out to the entrance/bus stop/comfort station.
It's impossible to name a single loop that is "best". It depends on what's important to you. And even then, there is often no single agreed upon answer. It's not a simple question.
Bama Ed
PS - you can request a loop but it's not a guarantee...