Using a Nikon D5100 or Sony SR68 to shoot video of an event
bmargulies, There are few questions here:


Are you looking for a multicam setup?  If so the two cameras are unlikely going to match unless you are willing to put in time to post production work. (fix it in post! http://d3j5vwomefv46c.cloudfront.net/photos/large/639498971.jpg?1345259967)  
What is the purpose of this video If its a 'creative' project (as in you are making a video about the event and not 'of' the event) then you should go for a camera that gives you some aesthetic flexibility which normally starts with a good eye and a good lens.
What is the budget for the shoot? If you have a budget then you should get the setup for the purpose.  If its a documentation then consider hiring a couple of cameras of the same model for a muticam setup.  If its creative then I would check how flexible your Nikon is to get the story/feeling/looks that you want to achieve - then see if you need to hire something better.


I'll also touch on what you mentioned about light levels.  If you are working with low light then here you would have two options:


Use the budget camera: DCR-SR68 and other off the shelf budget cameras will work well but at the cost of image quality (ie. the footage will look just like its budget)
Add flexibility to lighting - 1) get a LED Ring light, if you have a decent budget then you can hire out a Gekko George light.  2) You can also hire a lens that have a really low f-stop, just one stop can actually make enough of a difference. 3) Get a camera that can shoot RAW - RAW footage gives you more flexibility in post to push up the levels to brighten the footage and merry this with a decent noise reduction plugin and you will be looking at decent footage.


Hope this helps!