I don't have So Kate booties, but I have two pairs of Zermadames which are quite similar. I'm pretty consistently a 39 in Louboutins, but I have one pair in a 39 and the other in a 39.5. The larger size is definitely more comfortable, especially as I like to wear thin socks. The extra room is negligible lengthwise but the extra room at the instep is so much better for me. Don't worry about heel slippage, as they're boots. I think you'll be just fine with the 39.5!