Hi joebk,
I don’t have any personal experience on either mattress but as far as I can see neither one of them list an edge support system in the description. I would guess that the Bonnell probably has firmer edges but the only way to know whether the edge support in either one of them would be suitable for you would be based on your own careful testing or personal experience.
Both of these mattresses also use 1.5 and 1.7 lb polyfoam in the comfort layers which are lower quality materials that would be a weak link in the mattress that could compromise the durability and useful life of the mattress.
You can see my comments about choosing a firmer mattress first with the intention of adding a softer topper later in post #2 here.
In most cases I would avoid this approach because of the uncertainty involved with making two purchase choices instead of only one and choosing a topper that would be suitable in terms of thickness, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper (unless you can test the combination in person or you are purchasing both online as a “set” that is designed to work together and they both have a good return/exchange policy) and then use the option to add a topper as a “backup” strategy in case your initial choice is too firm and doesn’t turn out as well as you hoped for rather than a “primary” strategy.
Phoenix