It depends on whether you want to know the depth of the item. For me, I wanted know about where it was before I started, thats why I bought the Prizm III. but essentially they are the same machine except for the depth gauge. you have to decide for yourself whether or not the gauge is worth the extra money, to me it was. I think that you would be happy with either one.
By reading different posts on this forum and other forums, looks like the depth is the same on all the Prism's, just the bells and whistles are different on them.
I was going to buy a Prism III but since they only offer 2 coils, the 950 and the 8 inch, I decided on the M6.... more coil options and a lot better depth. I am not regretting it at all.
But if you just want a Prism to learn detecting, the III would be it, For the depth reading.