The colour one is a bit flat and lacking in contrast, which is there in the mono image.
I'd be inclined to use the colour one but adjust the levels and maybe darken the pillar as suggeted by the first reply so that she stands out more from her surroundings.
Thanks for the input. My original intent was to go greyscale - the shot was sparked after we had dinner the previous night in the restaurant in Shepherd Market that Garbo used to frequent when in London and which has loads of her pictures on the walls - but when I looked at the colour version of this shot I began to have doubts. I also considered monotone/sepia, rather than greyscale, but I don't think that worked. Maybe I should go down the duotone route instead.
Jitka prefers greyscale, so that's probably what I'll stick with.
It looks quite different (and in my opinion better) if you crop it tighter. The bit of bay window and the writing on the arch spoil the composition, but also add unnecessary highlights and shadow detail.
I normally prefer B&W but on this one the colour seems to work better for me. I think it is because the colour balance on the model and pillar are much cooler than the yellowish background which helps to draw the focus in to her.