Dover is indeed a reprint of the Durand, so I would simply buy the Dover. Even though I would not buy it, check and see if your library has the Wayne Leopold Edition, which is good one to consult, as he did a lot of corrections etc.

The corrections incorporated in the Wayne Leupold editions are extremely important.
The Durand edition contains numerous errors, while Dupre corrects them in his edition (but it has its own issues).
There are also errata sheets circulating for those who need to correct their Durand/Dover editions.

Gavin,

Here are my thoughts, and your options:

1) Buy the Vienna Urtext or Leupold edition. These are the best scholarly editions available; the Vienna also has wonderful physical presentation and materials (unlike the WLE). They are expensive, but remember: you will use this your whole life and the music is a treasure. It's as if one did not own a Bible, but printed out parts from the NAB website or something: it's just not the same with a cheap edition.

2) Buy the Dover, or, better yet, get CD Sheet Music's French collection and print it out. Then, and this is a must, go to a library that has the WLE and Vienna Urtext and NOTE ALL THE IRREGULARITIES!!!!! I studied with David Craighead on a few Franck pieces; his personal score had many meticulous markings from Langlais and Marie-Claire Alain regarding their notes on the performance practice and errata. If you're serious about the music, you need to do that.

It is useful to mark Dupre's opinions, but as DBP says, he does a lot of editorializing (articulations changed, etc., to match the "Dupre school" of playing.)

You should snag a copy of Rollin Smith's two masterworks on Franck, "The Organ Works Of Cesar Franck" and "Toward an Authentic Interpretation of the Organ Works of Cesar Franck" to make notes for any edition. He has many good suggestions, not to mention corrections.