As far as the standard plastic Bachmann knuckles go, yes, they're a straight swap on LGB stock. Cannot comment on Piko stock as I don't have any.
However LGB stock doesn't have a suitable small loop/hole to take the flexible "stalk" on tthe rear of the knuckle that's used to self-centre it. I tend to just cut the stalk off and let the knucke flop from side to side, but alternatively it's possible to fit some sort of loop of metal wire to retain the stalk.
Not sure about the metal Bachmann knuckles, the few that I've had supplied with locos have a different mount. I'd hope that they are available with a standard shank as they are better couplings.
Generally speaking I prefer the look of knuckles to hook and loop, but do remember that there are compatability niggles with the different manufacturers' knuckles whereas hook and loops all seem to work together fine. With knuckles it's best to standardise on one brand. I use Bachmann, with a few Aristos that I've fettled to improve the fit with Bachmann.
I also keep a couple of converter wagons/coaches with knuckle on one end and hook and loop on the other, for use with unconverted stock and/or friends' stock.