Hi glennk,
The Novosbed Aria is similar to the Tempurpedic Cloud Supreme and the Harmony is simiilar to the Sealy Optimum Inspiration so you can try these mattresses locally to help you with your choice between them.
Other than that … as long as you aren’t in a higher than average weight range (where I would tend to reduce or minimize the amount of 4 lb memory foam) there would be no weak links in either mattress (see post #4 here).
Novosbed is one of the better online options that are available to you (which is why they are included on the online memory foam list in the tutorial post) and they have a great return policy and would certainly be well worth considering as one of your finalists or as a final choice.
NOTE ADDED JAN, 2016: They have now introduced their new simplified choice mattress which has replaced their previous models and have also become a member of this site.
Phoenix