The benefit of PS5 Pro is that if you want to play in 4K you would need to spend 3-4x the price of the PS5 Pro to build a capable PC able to match the PS5 Pro in the GPU department.
An additional benefit is that if you have a disc version of the PS5 you can buy second hand games and play really on the cheap. You can easily resell your games afterwards as well.
The Deck is great, but it cannot run any 4K game and it is mostly to allow you to play your games on the go or in bed/sofa.