Go ahead buy it. From your posting this airplane will not be a regression but instead allow you to progress. The Three channel Senior is arguably the most successful trainer in RC history going back all the way to the seventies in continuous production. I wonder why that is?
Two of the folks I am currently working with: one has been flying sort of for five years with a large assortment of inappropriate aiplanes and cannot really fly. I recently set him up with a three control Senior and in just perhaps ten flights he can finally fly safely by himself and loving every minute of it.
I also now just taken on another who has had around ten airplanes every one of them wrong for him and he never really learned how to fly either but I,ve finally got him into looking for the right airplane after trying other Seniors and he is now looking for one. Unfortunately currently you cannot find a Senior arf anywhere and will probably be another month before they are avalible agine.
Whether or not you fly a three control or four control Senior is completely imaterial, It will not make a differance in your primary training but what will and is far more important is your Mentor/Instructor.
By the way adding ailerons to a Senior is not very effective unless you also reduce the dihedral for the reasons pointed out by Mr67Stang.
John