I have done both. I don't bag due to expense and possible catastrophic results if the seal breaks and the resin kicks.
They lay up the same.

Balsa is a great core material. It gets a bad rap due to bad installation. This is the re-core I did that is on the IHR: http://hydrostream.org/ArticleArchives/AlaskaViperResto.htm

I took a lot of time documenting the whole process explaining what materials and why they were used.

I have found that most guys on this particular forum tend to go balsa after doing some reading about the core materials. It seems the balsa can be a very good core material when laid up carefully. I am also interested in the foam core, but I found it more affordable to do the balsa. As far as vacuum bagging, you might find more input on screamandfly.. Sorry I didn't provide any good advice for the foam core, I haven't had that much experience building boats (Just what I have accomplished so far on my viper).

I am re-coring my 74 Viper. I may use a foam core. Can it be done with-out vacuum bagging? How hard is it to do compared to a balsa core. Does some-one have pictures or video of it being done.