Thank you for all the debate and all the links to read.
I have been in France, but return home to UK tomorrow. I don't know if I will find a shop that will let me try lenses out in the way the shop in France let me try the cameras.
For the theatre shots, I will always be at a distance of at least 10m from the stage and I will be setting focus and exposure manually. The Camera will be on a tripod and, as previously discussed I will have enough depth of field to keep the whole stage depth in focus. I have to have it set up in a way that means I can take photos without thinking about it, because my job is to be tweaking light levels, talking to the director etc.
I will also be nearly always using high ISOs, 800+.
At this level of ISO will I be able to differentiate the sharpness between the lenses anyway?
If not, I will go for the cheapest option and get a 50mm f1.8 as well (to cover me from the back of big theatres)
If I will notice the difference at this ISO I will consider them more carefully and perhaps try hiring before buying.
For family and holidays, it sounds like a good focus lock, and a wider angle will be beneficial.
Hmmm you have given me a lot to think about and I am finding myself getting mildly addicted to camera/lens research...