Tough one! My Bianchi is a 56cm, but at 6' it's perfect for me although a sizing guide would probably suggest the 58 or even 60cm frames. At 6' I'd go for the large in this Planet X bike, but if I was straddling the sizes, I'd be included to go for the smaller option rather than larger - unless you've got particularly long proportioned legs. E.g. if I was 5'11'' I'd probably still go for the medium, and likewise if I was 6'2" I'd go with the L rather than jump up to the XL.
I think (and I've probably read this somewhere too) that it is easier to make a smaller bike bigger through adjustments etc. than make a bigger bike smaller - if you catch my drift. Hence I'd go with the smaller size if I was on the cusp.
I spoke to the guy at Planet X and he seemed to think I should go for the XL based on the fact I have a longer body, rather than legs. That seems to be the opposite to what you're saying?!