2cm=20mm, approx. 3/4-inch. I've experienced 2mm of adjustment turning a marginal ride into a sweet ride.
That said, if you're being professionally fit to a bike, you need to get the right size. Otherwise, it's your cash and your ride. Try the advice vs. your own thoughts, and go with what works better. If a 58cm bike feels good to you, get it. (I personally would look hardest at a 59, cuz that's what my known measurements tell me would fit. But Brand A's 58 or 60 could meet those numbers, too -- seen that happen!)
my wife thinks so....i will take all the help i can get!
oh, regarding bicycles....when it comes to fit, go with what is most comfortable, not necessarily what bike size is "right" for you. if you can, wait a month, try the larger bike, it might be more comfortable, it might not. again, if you can wait, have as many choices as you can.
How did you come to the conclusion for those two sizes? If the bike is 2cm too small, the difference can be made up with handle bar location (stems) and seat location, but if the frame is too tall for you, there's not a lot to be done. Nothing will beat actually trying the two different bikes.