You can not compare both these exams. If you belong from Karnataka then you can appear in KCET otherwise, you can appear in COMEDK.

Through, KCET you can take admission in government and private colleges of Karnataka. To appear in KCET you need a domicile cetificate of Kanataka. Through, COMEDK you can take admission in private colleges of Karnataka. All the students can apply for COMEDK exam. 

Good Luck!!