I've owned the 552 (M390), 550 (HG - 154CM), and 550-1 (20CV). Still have the -1, have sold the others.
I could never get the edge on my 552 'just right.' For my uses, if I sharpened the edge low enough that it bit (into wood, for instance), it seemed really chippy; if I steepened the angle to mitigate the chippy-ness, it didn't seem to cut. The blade geometry on the 550 works much better for me. I'll agree, however, that the thin grind on the 552 sliced better than the fatter 550 (think: apples, carrots, etc.).
I still have a beat-up set of AWT scales that are just waiting for some Grip or another to fall into them! Thing is, I don't know when I'd ever carry it - I like the G10 scales on the -1 so much.
Anyways, I wouldn't beat yourself up too much about having missed the 552. I'd buy someone's user, but I wouldn't pay premium; hindsight being what it is, and all... Instead buy the 15400 and call it a day!