Why would someone buy the 7 rd mags over the 8?
What is the advantage or benifits of the 7 rd?

Quoted: The follower on the 7rd Wilson mag (#47) has a longer front leg that prevents it from “tipping” inside the magazine body. I personally use 7rd Wilson mags for most everything I do and have done so for years. IMO, on a long enough time line they may be more reliable. How long of a time line is another question. Nothing wrong with either the 7rd or 8rd Wilson mags so long as they work with your ammo and your gun.

Best advice, but what else would you expect.

I personally have never used anything but 8 round 47Ds for as long as I've been shooting and carrying 1911s. But as Jason aptly stated, use what works best for you.
As an aside, the Yost-Bonitz 1*s ship with a 47D.

I use nothing but the 7-round mags in mine, because the 1911 was originally designed for that.
(what was the recent discussion on here about the bump in the middle of some mag followers for 8-round mags, because changing the original design caused a new problem?)

Man, this comes up often - I need to save my response .

I only use the 7s for these reasons:

1.  I called WC and asked how they managed to get 8 in a 7.  They said they changed the follower to concave vs convex, and removed one coil from the spring - effectively making the spring weaker.

2.  Guy told me if I wanted absolute reliability, stick with the 7s.  

3.  I don't like the way it feels when you seat an 8 rounder on a closed bolt - you know - you have to compress that 8th round down into the mag.  Purely subjective, but I don't like it.

I've been using the 7s for years and had no plans to change - I just wanted to hear how they did it.

Quoted: As an aside, the Yost-Bonitz 1*s ship with a 47D.

Mine came with a Novak mag.

Back on topic... I would go with the 7 rounders for the same reasons as Backstop, except I like the old fashion 7 rounders with welded baseplates because I don't like to see those thick plastic base plates sticking out of my pretty gun. Even the new thinner "concealment" bases are still to thick for me. I like Colt mags or Metalforms or Mec-Gars.