I'm not as fancy, nor made of as much money as some of the other board members, so I don't know what stock you can take in what I say BUT, here is my two cents.
My wife got interested in detecting with me recently. I thought about purchasing a really cheap POS just to get her one, but ended up buying the Whites Prizm III, and upgrading the coil from the stock to the 9.5 inch.
The Prizm IV is the same price as the III with the upgraded coil, but the only thing you get different is th 3 tone ID and the smart notch. I don't know about anyone else, but I don't like the smart notch, all it does essentially turn the sound off of certain things, but it still shows on the digital read out. The 3 tone ID is something I think she can live without.
So, I suggest ordering a catalog from Whites, it comes with a coupon for $50 off a Prizm until the 30th. After reviewing the Prizms, I realized that with each number (II III IV etc etc), it increases about 2 inchs roughly. The new coil adds depth by itself, so I just went with the middle of the road. If your on an extreme budget, you can get the Prizm II for $199 right now with the coupon, and upgrade the coil later.
My two cents.... Everyone else, flame on I'm sure.