From my point of view PayU is suited for large business only. But small business can also use PayU to collect payments from their customers but again, I think PayU is suited for large business mainly.
If i have to pay the amount via UPI then i will prefer PhonePe this is my prefered platform, But if i have to pay on the Paytm then i have the Paytm postpaid limits so i will pay the payment via Paytm postpaid. I think maximum users has Postpaid limits on the different platforms like Amazon, Paytm etc.
Payu is a much better and easier to use platform. It doesn't charge any upfront fee or annual fee. The payment processing charge is very low. The remittance is very quick and there is no minimum amount criteria for payment remittance. Payu offers tons of payment options. The best part is the payment page. The page is simple and beautiful
The only thing that I find as a reason to use phonePe is the ease, comfort and convenience in doing what you want from a payment app. You get multiple competitors but you need to select for the best UX