Let me just add to the other answer:
Plagioclase feldspar is the most common mineral in the Earth's crust. You will find it in most rocks plutonic rocks, and it is also a very common mineral in volcanic rocks. You will find it in gabbros, diorites, andesites, most basalts, most granites, some rhyolites, all monzonites, some peridotites, and list goes on and on.
Alkali-feldspar isn't as common. The only common rocks which contain alkali-feldspar are most granites and their volcanic chemical equivalent, rhyolites. Other than that, you will find alkali-feldspar only is more rare rocks such as syenites and other weird rocks that you will probably never see or hear about.