1) Capability - processing power, storage, RAM size you need to run the processes you desire;
2) Ruggedness - whether it has to be durable for challenging conditions (water, tumbles, dust)
3) Battery - battery life on single charge, hot-swappable or not.
4) OS & Connectivity - needed Android version, 4G/5G, Wi-Fi, Bluetooth, NFC
5) Additional features - display type (capacitive or projective touch), physical keyboard, availability of accessories (pistol grip, charging station, RFID reader etc),
Read more about choosing a handheld terminal from here.