I don't think rust is really the factor, since old Peugeot rust like old Citroën (some Citroën don't rust, like the XM, some were rusty as soon as they quit the factory, like the early CX) or old Renault, I think it is mostly a question of reputation, and maybe design.
People almost never criticize old Peugeot here ! And their rating is higher than equivalent Renault/Citroën. They got some nice designs, the 205/405/406/505/605...look classical and still elegant...some Renault can't say the same, like the R5 "supercinq"/R18/R19/R21...seriously !
Citroën are a different thing, there is a lot of "Citroën clubs" with people going crazy over what we call "hydraulic Citroëns" ! Even some magazines about Citroën only. But you don't see these collectors on the road, only once in a while! My friend has a BX 16s (1905cm3 - 160 hp), and I have to say, in the France of 1986 it must have been a really exciting car.
Maybe also the customers of the different Brands, Peugeot was regarded as a very serious brand (before the 80's) and it kept this image (maybe not now however), I know lots of old people with old Peugeot.
And then we can talk about engines, Peugeot/Citroën had some fun and reliable engines for the time, like the GTI engines, declined with 16 valves and/or turbo, some lively little engines like the 1294 cm3 in the AX sport and 106/205 Rallye, or the 1360cm3 with twin carb, even the turbodiesels were refined and reliable (at the time) and sold to Rover. Renault had some antiqued OHC designs, sometimes unreliable, even some unreliable diesels. And the newer engines with OHC were not really fascinating (not talking about the 1,8L 16s which was lacking torke or the 2,0L 16s in the Clio Williams which was rarer as hen's tooth !).
Just my opinion here, but if I had to pick a French car from the 80's or 90's, I'd most probably pick a Peugeot or Citroën over a Renault. In fact when I bought my Barchetta, I was looking for an AX Sport