I didn't want to create a new thread, and I didn't want to put 'sales talk' in the official thread, but MCV has a report here on UK retail opinion on GTAIV, and relative platform performance. People sensitive to platform comparison etc. may want to look away now!
It seems polling retailers on the platform split is fashionable these days, but this report has more than I've seen elsewhere.
HMV
Asda
Comet
GAME
This is taken from the lead article in the print edition of MCV.
http://www.mcvuk.com/print-edition
I saw another report quoting a buyer at GameCrazy in the US, saying that interest was split a lot more evenly as with more recent releases like DMC4. Play.com preorder charts usually have the two platforms switching positions too. Ditto with Amazon.co.uk.
Of course, these aren't ultimate indicators, but they're probably the best we can do for now.