One must for me is a mattress with a reasonable trial period. I want to make sure that I'm really happy with it before I commit long-term. So I won't buy any mattress without trying it.
Are you saying that spring mattresses vary from unit to unit in the same product line? I can't see why spring mattresses would be more apt to have unit variations than foam or other matteresses.
I tried a locally made spring mattress with Dunlop latex topper and really liked it. However the company has no trial policy so I'm trying others now. Who knows, maybe I'll come back to this mattress.
I agree that mattresses are very individual. Some mattress companies allow customization for each side of queen and larger mattresses. That would increase the chance of having a shared mattress without compromise.
The Parachute looks interesting and has a trial period, so it will be the first I'll try. If I love it at the end of the trial period, I'll buy it otherwise I'll chose the next mattress to try.