Having used both jqGrid & slickgrid in production/anger, here are my 2 cents:
jqGrid was very easy to get started with and hook in. However we quickly found ourselves grating against having to fit what we wanted into jqGrids way of doing things. We also found the editing capabilities difficult to implement and (from memory - this was a year ago) it forced you into a row edit model, rather than cell edit which did not perform well.
With slick grid we have yet to hit a feature that was missing / we could not easily plugin. The documentation was fairly non-existent but is now much better (https://github.com/mleibman/SlickGrid/wiki/_pages) but to date we've just been jumping into the source. Have to say the api is one of the best I've seen, and performance with large data volumes and complex editors has been excellent (some issues on ie7 but never isolated that to the grid vs our MVC framework & complex editors)
All in all, I'd highly recommend slick grid
