Speaking for myself, I would prefer the original chambering since both the 369 Purdey and the 375 Flanged are handloading propositions and both use the .375" bullets.
I can see the neccessity of a rebore/rechamber in barrels which are badly corroded, but a rechamber of a classic double while leaving the bores alone I don't like as much especially in two cartridges which don't have that much practical difference between them.
Not like the .375 Flanged brass is any easier to source today than the 369 Purdey anyway.