To me, this is an easy and clear choice: 527.
In many ways, the two are similar, or even equals. Weight, size, corrosion resistance, etc. I suppose you might get a touch better edge retention with the 556, and it is a tad smaller, so it might fit your pocket a little better.
But with the 527 you get full liners, a stronger pivot, handles that will probably treat your pants a little better, and a fuller four-finger grip. Also, you get a hollow ground blade, which I've found to at least match the cutting efficiency of my 555 (non-HG version; essentially identical blade geometry to the 556), if not exceed it in pretty much everything. It can feel a little different than the flat grind of the 556 at times, but I haven't noticed anything where the hollow grind gets in the way.
I used to be of the opinion that the Mini-Grip was about the best all-around knife for cutting tasks, striking an excellent compromise between affordability, quality construction and general utility. But now that I've got the 527, I believe it one-ups the Mini-Grip in just about every way.
Mind you this is my own opinion, and others may find the knives work differently for their use. In the end I'd recommend the 527, but really, both knives are winners.
