Sounds like an epic battle...or not but I can now offer a little perspective on both, pros, cons, and personal preference. There are substantial differences between the two and I went into my testing thinking that practice skin would be far superior because it was supposes to mimic skin, but after testing both "products" I have come to a different conclusion.
Practice skin is manufactured by a few companies and mine came from MOM. Upon opening the package I found it to be a little rubbery but not nearly as flexible as I had hoped. The one cool thing about this is that you can rubber band it to sections of your body if they aren't too curvy so that you can simulate tattooing on a specific area which is a huge plus. As I started using it I noticed something right off the bat, the ink didn't seem to take very well (this could be my lack of experience and being a total newb), this bothered my greatly because I was having to overwork areas. One of my problems I think was after reading the Guy Aitchison manual he talked about running his machine at as low an ampage as possible so that he can make lots of passes over it. I had been working it at 6-8 but when I cranked it to the 10-12 range I found it going into the practice skin far easier.
Now for the grapefruit I didn't make a stencil to transfer, just freehanded a nautical star. One of the more inherant problems with grapefruit are their shape which makes stencil transferring pretty difficult...plus I suck at it still. I personally don't like the shape of the grapefruit because even for small designs it obscures your lines from your vision, it makes the process a little slower because you have to readjust so often. Despite these flaws, the ink goes into the grapefruit effortlessly... and even more so when you clean up your work you don't lose the ink as much.
The problem that I now face is which is more accurate? You can't practice your stretch on grapfruit but I'm pretty sure it's a more skin realistic surface, I'm going to continue working with both because I need to build up handstrength, and practice is practice but this is one of many reasons I am discovering I need to get into an internship fast, I have too many questions that my materials don't answer, and that other people won't help me with.