I slept on a 'fake' futon, similar to the one linked (8in, coils inside cotton batting), for ~10 years. I loved it, and feel like it held up very well. Everyone who has slept on it gave it good reviews. It had very little/no bounce, let you sink in a little, but no further. I feel like it would have held up longer if I'd been more contentious about flipping/turning it. I still have it as a guest bed, but the middle where I slept eventually had too much of a dent for me to sleep on it every night. It was definitely on the firm side, which I prefer, but not something I'd categorize as terribly firm.
I tried the internet foam mattress recommended by the Frugalwoods that that is an Exceptionally Firm mattress. It made my hips and shoulders hurt (I'm a side sleeper).
I eventually settled on the more expensive foam mattress from Lessa. If I could have found something more similar to my old mattress with a decent return policy, I'd have done that.