Sofa bed advice needed

[NewMamma21]

We're looking to buy a sofa bed for the spare room / office.
I used to think one with a pull out mattress would be comfiest but we've stayed on a friends and it wasn't comfy at all.
We like the idea of a corner one (with one side that opens up for storage) but do they work okay to fit sheets on?
I'm thinking i'll need to buy a topper if it's a fold down / pull out one without a mattress so you can't feel the indents of the different parts / cushions of the sofa.
Thanks!!