There is no "best" - Samuel teaches using Derrick's notes / method.
With Samuel it is private tutoring but with Derrick you're in a group setting. If you struggle with methods Samuel can dedicate the entire lesson covering a topic that you need assistance with. Derrick still helps with any specific questions you have but naturally his time is limited as he is also helping other students.
I'd say if you want to move at a different speed to your cohort then go with Samuel. He has a lesson plan (prepared by Derrick) each week but he will tailor the lesson to what you want. You don't have the same flexibility with Derrick.
I started with private lessons with Samuel but transferred to group with Derrick to reduce travel (plus classes on a Sunday were easier than a weeknight). This worked for me because I really did not have specific questions each week that needed individualised help from Samuel. It was still worth getting the tutoring though (it's expensive) because Derrick teaches things that were not taught by my class teacher (or if they were, he teaches them in a way that I could understand).
As appleandbee said, unless you've been on a waiting list for Derrick for a long time you probably won't be offered a place with him - Samuel is in no way an inferior option though. He was tutored by Derrick, teaches using his methods and probably most importantly has the ability to explain concepts well (and the patience to go over them again and again if necessary until they "click").