RE: Whey Protein Powder or MRP Shake?
MRP does it for me, I bought a large tub of Sci-Mentor advanced MRP with biotics and was very impressed. I'm a fairly hard gainer too (but don't put on fat), so the MRP did the trick. I added half a scoop of whey to up the protein content.
I'm now trying some weight gain power (Sci-Mentor too) with an extra scoop of whey, and am gaining about a pound a day right now!!! Of course, it may have something to do with the Methyl Drol I started at the same time ;-)
I'd recommend MRP as it's got most of the extra vitamins and minerals you need, and only a trace of fat. If it's got pre and probiotics in it, all the better.