Hi,
I think the difference will greatly depend on your workload indeed.
If you need the memory, it is a simple choice as there is no way that I’m aware of to merge two cards together.
The biggest benefit I would see from having two cards though is that when running cross validation (or any parameter search). It is much simpler to run two job at the same time. And this will most likely be faster than the single card (as long as you have enough memory on each).

This leads me to the next question, is the increase in speed on the titan just down to the increased memory size?

No I think it’s unrelated. The die is just bigger with more compute cores.