I am actually struggling a little with the question. I have no idea what roughly half the species would prefer, I can only speak for myself. You know how people vary? Well that probably includes women. There are a lot of them out there, all over the world, probably not entirely homogeneous.
I think for most women a larger white diamond will be preferred. However, I think this is the case because the pink is as you say much smaller. I think a girl will choose a Pink diamond and not care about the size if she REALLY loves pink diamonds and the idea of a pink diamond... but I also think she might choose that if the size was at least 1/2 carat or more. Have you checked out amgad.com? I saw a pear shaped pink diamond on there before around 1 carat for a very reasonable price. Also try purple diamonds, they can be very close to pink in color.
Much as I love pink diamonds, I think that the size will make too much of a difference- so I'd vote white.
If it was a right hand ring the pink would have a much stronger showing.......
Elle from Legally Blonde? Well... I'm getting a pink diamond E-ring. I guess that makes me Elle Woods.
Still if it comes to size, I'd prefer a bigger white stone over a smaller pink. If they were both the same size, I'd choose the pink. But what do I know, I'm a bimbo
Larger white stone, without hesitation. I would be very upset at receiving the .20 pink stone, regardless of how nice it was, for an engagement ring. Put in a halo and make it into a pendant and I would be happy to have it