There's no rule as to whether powder or rock will be stronger. Generally though, crystals only form with relatively pure product, but there are cutters that also crystallise and smoke well; there are also methods to combine crystalline methamphetamine with other cutters so you'll end up with an unhomogeneous shard.
Rocks are always medium to low purity - 30% or less pure (usually much less, considering the average purity of meth is around 5-10%.) As pure meth is crystalline, anything causing it to be able to form into a rock indicates high levels of impurities.
Obviously, powders are very hard to identify and powdered meth could be anywhere between 0-100% meth.
The only way to know your purity is to wash your product with a simple acetone wash. I know I would much rather be inhaling as pure meth as I can get, because who knows what a potential plethora of impurities could be doing to your lungs (as well as for obvious reasons ). Meth inhaling in itself likely causes harms as well, but cutting down other adulterants is still very likely to reduce harm.
doesn't matter for me, as long as it melts into thick oily goo puffing out tonnes o smoke. As far away from a watery consistency as possible the better.