I've been doing OpenGL programming for some years on both ATI and Nvidia cards, I don't know exactly the open source policy for both companies, honestly, as long as they keep a good and working support for Linux, I don't really care, what I can say for sure, is that both of them share a good amount of knowledge, advanced programming techniques and tutorials completely free.
As far as I know neither ATI, nor Nvidia open sources its drivers.
Another important aspect is that Nvidia has by far the stablest and most standard compliant implementation of OpenGL, with the Nvidia drivers, you simply are sure that it works, programming on an ATI card you will often come to bugs or inconsistencies in the API, of course, they will eventually fix it, but there are issues and, in my opinion, there are too much of them.
On Nvidia platforms, by contrast, if there is an issue, it is 99% of the times your own fault, to quote John Carmack: "Nvidia's OpenGL drivers are my 'gold standard', and it has been quite a while since I have had to report a problem to them, and even their brand new extensions work as documented the first time I try them. When I have a problem on an Nvidia, I assume that it is my fault. With anyone else's drivers, I assume it is their fault."
I don't know very well the Optimus Technology, but a fast reading of that technology, seems merely another name for GPU switching when graphics performance is not needed, if this is the case, it does not work on Linux simply because Linux is unable to do it... Yet
this is being worked on, see:http://en.gentoo-wiki.com/wiki/Vga_switcheroo