Ok, perhaps this hasn't been the most informative thread to come along in a while but I'd like to make some general comments. First, the 8mm difference between 700C and 27" is so small that I'd dare anyone to notice it in a blind test. That said, Kurt is correct in that vary few 27" rims/tires were designed for performance applications back in the day. In the hayday of 27", the 60's and 70's, performance bikes almost always used tubular rims and sewups (yeah, I know you could special order a Paramount with 27s and stuff like that but I said "almost"). By the time good performance clinchers came along the move to 700C was already well underway. Additionally many European bicycles from the 60's and 70's that came with 27" rims (typically entry level to mid range models) were not designed for 27" rims but rather had that size rim installed for export to countries using English measurement standards. Take a look at the chain stays on a typical Peugeot UO8 where they are indented for tire clearence and you will notice that the indentations don't line up with 27" tires but do line up with 700C tires. That's because the frame is essentially the same as a French P8 models that didn't use 27" tires. Frank Berto, in his 1988 book 'Complete Guide to Upgrading Your Bike' recommended 27" for touring bikes not for performance reasons but because in a pinch you could find a replacement tire at any hardware store in BF Egypt should you be out on a tour while only a bikeshop would have 700C and there might not be a bikeshop in BF Egypt. For similar reason he recommended using 36 spoke rims (as opposed to 40 or 48 spoke) on touring bikes because any bike shop would have them in stock in case you bent one.

Personally I prefer 700C because of the tire width selection. If I want to run 23mm on a set of Mavic MA2 rims or a 35mm on my Rigida AL1622s or 38mm on my Fiamme yellow labels or some wide (wider than 1 3/8) knobby cross tires or even mount some studded snow tires on a bike (like I'd like to do this winter on my Panasonic but can't because it has 27s) I can easily do it with 700C. With 27" I'd be much more limited and generally have a harder time finding the tires even when they are made. Of course if you have a frame that will only fit one size or the other or already you have decent rims and no reason to change them then I don't see an issue with using either one.