Either or, though if you're interested in an academic job later on and want a lot of research experience, WUSTL would be the one to go to. They have more resources and connections that can get you into papers and presentations more readily than SLU can. Research at SLU is hit or miss -- you really need to look up your PI beforehand to see how prolific they are, and if their pubs historically include other students/residents on them. Research is definitely something you can do year-round or focused on the summer. It depends on what you hope to get out of it and how gung ho you are about completing the research. I say that because I've had experience working with other SLU med students that jumped onto projects and pretty much piggy-backed without contributing much except for attending meetings.