Slow- I'd go with the 39s. My KSOs fit snug, but not uncomfortable and I haven't had any problem with them. I've even managed to wiggle injinji socks into them for winter running and while the toes were very full it hasn't been an issue. With about 200miles on mine: the fabric breaks in a little bit, but I wouldn't say it stretches much further. If anything it's your feet that adjust to being wrapped in the VFFs the more you use them. Hth!