Do different Macs have different quality audio ADCs?
The physical 3.5mm input jacks are likely the same or nearly identical, but the relative quality will be determined by the audio chipset used. I would check System Report on both machines, go to Audio, and see what chipsets are being used. It may not show you enough detail to know exactly which version it is, but hopefully will provide enough detail to research their respective specs online.

I wouldn't use the 3.5mm jack though. Since you have a Mac Pro you should have an on optical input available. This will be much higher resolution than the mini jack - 24bit/48k vs 16bit/44k (in most cases). Take advantage of that with something inexpensive like a Behringer UCA202 ($25 I believe.) The ADC's on it are designed for digitizing vinyl, and it has an optical out that you can connect to your Mac Pro, for the cleanest possible signal transfer to your recording software. Most people who digitize vinyl do it at 24/48 because of the added headroom it provides.

(Edit: I should have asked if your turntable's amp provides RCA outs, because that's how you would connect it to the Behringer. If it only has 3.5mm out, and you don't want to purchase a separate preamp, then your only choice would be to plug it in directly to one of your Mac's 3.5mm jacks. Just realize those are very low fidelity inputs without much amperage, so you will need to do major volume boosting in your recording software to compensate, which will introduce noise.)