Returning after a lay-off, Canondale frame sizes vs Giant

Scottster72

Hi, this is my first post on this forum. Now I'm getting to that age where I'm looking to recapture some of my youth, I've decided to take up cycling again. I was a keen cyclist from my early teens to my mid 20's, but from then university, an active social life and then marriage and kids have all kept me off the road - until now. I am looking for some advice on getting a new bike as things have moved on somewhat since I left the scene. Back in the day the only bike i rode was a Reynolds 531 frameset with mostly Shimano 105 group set (lack of funds prevented me from upgrading or buying other bikes!)! which I clocked up a fair few miles on. I've been eyeing up a 2011 model Canondale CADD8 CD (part shimano Sora) and was on the verge of taking the plunge, but I've spotted a Giant TCR2 (aluminium framed), which is a slightly pricier bike but due to a bigger discount the difference in price is tempting me to go that extra step, nearly full Tiagra group set and a nicer looking bike. Now I know the real advice is to ride them, but I've ridden a few bikes recently and really can't tell the difference just from the 10 minute ride up and down the road in front of the bike shop' after all, my only point of reference is my old bike, and everything I've ridden so far feels in a different league. I was going for a 54cm Canondale(I'm 5'9"), which feels great, but can't test ride a Giant as there are no stockists near me. According to the Giant size chart a medium is a 55.5cm frame. My question is; Am I a fool to assume that the Giant bike will be near as makes no difference the same size bike as the Canondale meaning a blind Internet purchase is relatively low risk? Or do sizes vary significantly from brand to brand?

Thanks.

I'm purely in it for recreational and fitness purposes, I.e. not competition, though I will be competing against myself.