I was going to suggest that the wheel size should not have that much difference, but, as suggested above, comparing a 700 X 25c to a 27 X 1-1/4 is not a good comparison... If the tires on both wheels had similar casing profile then the difference in wheel radius, which will affect the trail and therefore the stability, would be approximately 4mm. 25mm tires might be another 7mm shorter than 1-1/4 tires, so the radius could be up to 11 mm less than with a 27X1-1/4... that is a bigger difference ad more likely to be noticed while riding..

FWIW, I always though the two biggest advantages of switching from 27" to 700c were the availability of different tires, and the ability ot fit wider tires into the same space.


My current main ride is a 27 -> 700C conversion and I am using 700 X 35c tires and it rides and tracks like a champ.

However, the previous bike restoration (more like a revitalization as I am not trying to necessarily trying to return the bikes to any condition other than 'ridable') I left the 27" tires on for quite a while and I loved the way it rode. I was amazed at how fast I could cruise down the highway in a big gear even with el-cheapo $9 tires fromt he hardware store on steel rims. THere is certainly no disadvantage to staying with 27" if you can find decent tires that suit the riding you want to do.