I think both have their place, but you need to be careful... When I first started studying the swing, slo-mo was necessary because I simply wasn't capable of seeing what other people were talking about. Now I rarely look at slo-mo. In either case, if you are not careful, it can be easy to see things that are not there. The bigger issue is isolating one or two swing clips and making the assumption that each is representative of that hitter, or worse yet, representative of a high level swing. Was the hitter on time? Early? Late? What was the pitch location? High? Low? Inside? Outside? Sometimes it can be difficult analyzing a 3 dimensional swing with a 2 dimensional video.
The biomechanic guys say that you can't see what is actually happening unless it's a high frame rate. Yet some say that slo motion isn't good to look at to much.