I think you have to decide on a panel type first. For mid-range you can get IPS or VA panels. VA generally hs nicer colours and better contrast but can have black level smearing.
I really wanted to avoid black level smearing so I went for a flat panel one as this was the only ips I could find in my price range. For high end there are some good VA panels with very low/not noticeable smearing.
This is probably a more important difference than flat vs curved.