Jonro, you asked for it, so here it is:

My particular pens are a bit difficult to compare as they have completely different nib sizes: My PFM I has a medium palladium silver nib, whereas the Legacy sports a very broad right oblique 18K gold nib. Due to the materials used the Legacy nib feels much softer.
Both pens are very good, wet writers that will write on anything with any (FP- ) ink I've tried. Yet both nibs showed some skipping or rather hard starting on some downstrokes at the beginning, so I had to very carefully smooth away their "baby bottoms" (it found it weird that the new old stock Legacy had this problem and I tried all kinds of other remedies first, but it was really a 'baby bottom' effect that caused the hard starting/skipping).
The - all metal - Legacy is a tiny bit longer than the - plastic - PFM, and it weighs 35 grams capped and 20 uncapped compared to the PFM's 20 (capped) and 10 (uncapped) grams (sorry, I made a mistake here in the handwritten comments). I was very surprised to discover that the difference in weight that everybody always comments on when comparing these two pens hasn't bothered me at all so far. This must be because - like the PFM - the Legacy is very well balanced.
Finally, there's the difference in filling systems, and while the cartridge option that the Legacy offers can certainly be handy sometimes, I definitely prefer the PFM's snorkel system: it's mess-free and the parts seem to move more easily than the Legacy's touchdown mechanism (the Legacy has a special converter that can be filled like the sac of a touchdown pen).

All in all I'd say that both pens are really brillant writers that feel and look good too.
Nellie