Choosing a mattress is displayed in stores for a reason. There not just there to look pretty- they are there for you to test them, if you comfortable and your body is too. That the way you know which one is perfect.
Just by sitting in the corner of the mattress doesn’t mean you will know like that if that mattress is for you. You can’t possibly know by the edge of the mattress because, first you going to spend the majority of the time on bed not sitting down. Secondly you not going to lay down on the edge of the mattress, you are going to want to be in the middle.
And if there are two people sleeping in this same bed, you’ll need to lie down at the same time. Just look the one is comfortable and support for you.