I've only used vintage Gillette open combs and a few vintage German examples. I think one reason Gillette went with the bar guard was because guys dropped their razors, bent the teeth on the open comb, and if they tried to bend them back straight, many times they broke off. Also the bar guard must have been a lot cheaper from the mfg point of view.

I personally can't tell a difference in the shave between the two, other than the blade exposure Obie noted, and that is more a matter of the individual model razor than whether it is bar guard or open comb. IMHO.