I am having difficulty deciding on which field seminar and which topical seminar to take: can you help me choose?

The main criterion is that you should have a strong interest in the field and have some foundation in the field.  Most students find they work more intensely than they ever have before and to sustain that level of effort it is important to have a genuine interest in the subject matter.  You might choose seminars not available at your home university, or ones which depend on the extensive libraries and other research collections in Oxford.  It often works well to build on topics you have particularly enjoyed at your home university and would like to pursue in greater depth.