Old dual impedance SM58 or new?

[conanthewarrior]

Hi everyone, I have two SM58's that I use for recording. 

One I bought brand new, another is a used model from the late 60's to early 80's after speaking to the manufacturer. I got it cheaply, all it needed was a wire soldered back in place, it also has an extra low impedance mode which is used by connecting a different lead inside at the bottom. 

For some reason with the older SM58 there is a lot less pop type sounds and it is also less Essey. 

I have checked this blindly and it does sound better to me, but does anyone have a reason why an older SM58 would sound better than a pretty new one with no use? 

Also, I know the answer, use what sounds best, but would you use the older one more, or the new one for vocals? I know a condenser is usually used, but my budget at the moment got me the SM58's, and I know they are a great all rounder that can be used for professional use (U2 for example).