Hi. I have stayed at both Brasenose and Magdalen as part of summer study programs in the 90s. Both are great. Brasenose is more in the heart of things with the Bodleian right outside the doors. Magdalen has more greenery and some walking paths to explore. Not sure which rooms are available. Both have older buildings and newer buildings. BTW, Magdalen is not so far removed as to be a difficult walk. In fact, you may get to see more because you have the option of walking various lanes to and from the college.
Read carefully about where the rooms are and what type they are. Two summers ago, I stayed at Wadham. The room was great, the location central and beautiful, shakepeare performances in the garden. Unfortunately, a few busloads of teens were also housed in my building and they all checked out around 3AM. Much noise and pounding up and down the stairs in the hours leading up to 3AM. That experience - which I believe can be replicated at many of the colleges - has led me to try for the more expensive single ensuite options at some colleges in the hopes that busloads of teens will not be next door.
I understand the lure of seeking out a more traditional setting; just read carefully and google any buildings to be sure that is what you are getting.
Have fun. Debbie