I finally bit the bullet and made the leap from 5 to 6. Initially I was held back by the lack of support for my favourite blogging software, Marsedit. It doesn't look like Squarespace are going to put back the compatability with Marsedit any time soon. Eventually, I felt it would be easy enough just to work with Markdown text files and then copy and paste them into the Squarespace 6 post editor. It seems to work fine. I chose the template called Hudson.
The compelling reason to switch was the responsive design templates available on v6. Most of my traffic comes from mobile devices (phones and tablets), so this is important. I like the fact that I don't have to pinch and zoom on the iPhone to read the site easily.
I also decided to merge plaqueimaging.com and medbytes.tv into a single site with two distinct sections. I think it looks pretty good so far. Clean lines, no clutter. Almost iOS7-like. Don't laugh.
The switching process was reasonably easy. Getting used to the new system is happening slowly. I'm happy.