A 4A-GE 16 valve is an easier swap in a KE chassis Corolla than a BT/ST, not to mention it's easier to live with. Blacktops/Silvertops require more fabrication to fit and run right. Trust me, I'm in the midst of a project like that! That's why a 16 valve is rarer and more expensive than a 20 valve.
You can daily a 16 valve as it takes regular fuel, unlike the BT/ST which requires HOBC.