[Gogar]

I also am no expert, but I know a few things:

In 1973, Only the 911T came w/ CIS. I think there was also a version of the T with MFI (up to 1973.5).

E's and S's in 1973 were MFI.

So, your value guide may only be considering T's, because E's and S's are obviously much more desirable.

That said, in a practical sense, it depends what you are going to do with the car. If you're going to drive it a lot, the CIS might be a better idea. If you're going to go to the track or something MFI might be loads of fun for you. CIS parts are cheaper and more available, too.