Loretta and I co-lead the Atmospheric Chemistry Modeling Group. We have complementary research interests and approaches, as you can see from our web pages and publications. You can decide which fits your interests best. In practice, Loretta and I talk all the time and often share students depending on projects, so you can very easily end up working with the two of us. We also share notes about applicants so you shouldn't worry about compromising your chances to work with one by applying to work with the other. Students working with either of us belong to the same group, share offices, work together, etc. There's no distinction.