The new.
I'm not sure if by "new" you mean the PME51 vs the PME5/PME3/PME/VFC-6, or
the PME51 vs the PME45, but it doesn;t matter for the answer itself.
It does matter for the reason behind the answer.

The old PME series (starting with the VFC-6, culminating in the now
discontinued PME51) did evolve; problems, either potential and/or real, were
sorted; and the meters got better.
That's not to say that anyone of this old series was bad. On the contrary.

Compared to the PME51, the new PME45 is a more versatile meter by far.
It offers three metering modes (incident !, spot and center weighed
integral); offers more convenient adjustment of viewing; and allows many
parameters to be set by you, the user.

Both new PME45 and PME90, and the "old" meters from PME3 onward are
calibrated to be used with the Acute Matte and Acute Matte-D focussing
screens. Should you happen to be using an old style, pre-Acute Matte screen,
in your 500 C/M, you will need to adjust the meter by setting the ISO value
to double that of the film you are using (i.e. 1 stop).

I have one for sale.  If you interested contact me.
Meter Prism PME 42294, Mnt. $315.00