By default (on my mac anyway) it selects OpenGL,a few times i have had Carrara crash when coming back to the assembly room after changing a few shaders, i have reported this but daz hasn't done anything.

What i am wondering is what is the best one to use - OpenGL or software, and would changing it solve the problem i am having. I don't seem to have the shader to assembly room issue on windows.

Mac is up to date with patches and such

 I use both methods depending on what I'm doing. I haven't experienced the crashes you have but I will occasionally get delayed screen redraws when oGL. Usually, simply switching between the two resolves the problem. 

This is a Carrara issue because I have other 3D applications that do not show similar problems when in oGL. However, I personally haven't been able to reproduce it regularly which makes it a real problem to identify and report as a trackable bug. 

There is really no right or wrong way to use the two methods. I use oGL when working with transparency, hi def textures or doing 3D painting.

Will try software and still see if i get the issue, when it happens i cant do anything, sometimes i have to hit cmd+q to kill it, sometimes even that wont work and a kill -9 is required.

I save every 5 mins now when on Mac to make sure if i do lose anything its not going to be much

I'd suggest switching to software render just to see if you have the problem occur there or not.
Carrara is very stable on my Mac. I have never had what you describe occur to me.

i wonder if its beause i am using snow leopard, maybe Carrara inst as stable on there as 10.5.x, and yes i am going to see if software helps

Could be. I'm using Leo 10.5 - not in a rush to upgrade. Apple's been pretty good about compatibility lately, but each new OS release is bound to break something.

Hope you're able to resolve this,

i think i may roll back to 10.5.x and see what happens

After some more playing about, i think it may be a mem leak issue, i can get carrara to freeze on windows with only a few things going on, and the mem use went straight to 4gb without trying.