Hi Powagoat,
Firmness and softness is always subjective and relative to the body type, sleeping positions, and perceptions of each person. As you mentioned as well … with memory foam it can also depend on the temperature of the environment because memory foam changes in terms of softness/firmness based on temperature, humidity, and the length of time you are lying on the mattress. It can also depend on whether the mattress you are lying on has been broken in or is new (you can ask the retailer how long the mattress has been on the floor). I would make sure that you have tested both of these (or any memory foam mattress) for a minimum of 20 - 30 minutes to make sure the memory foam has had the chance to soften especially with higher density memory foam which tends to take longer to soften than lower density memory foams.
In addition to all of this there are also different types of firmness that each person may be more or less sensitive to (see post #15 here).
There are also two versions of the Allura … one of which is in the Cloud line and uses 4.1 lb memory foam on top (the softest memory foam they make but which is also faster responding than their higher density memory foams) and the other is in the Contour line and uses 7.1 lb memory foam on top and would feel firmer.
The Rhapsody uses 1.2" of HD memory foam (which is 7.1 lbs and is softer then their 5.3 lb memory foam but firmer than their 4.1 lb memory foam) over 2.8" of their 5.3 lb memory foam and as you mentioned is “rated” as medium firm. With a thinner layer of the HD memory foam on top the “feel” of the 5.3 lb memory foam would “come through” more than it would with the Allura.
The Contour Allura uses 2" of HD memory foam over 2.8" of their 5.3 lb memory foam so for most people it would be softer than the Rhapsody (it has a thicker layer of softer memory foam on top but it may take more time for the top memory foam to soften) but it may take some time for the softness of the higher density memory foam to become apparent. The “feel” of the HD memory foam would be more pronounced in this mattress than it would be in the Rhapsody.
The Cloud Allura uses 3.2" of 4.1 lb memory foam over 1.6" of 5.3 lb memory foam so for most people it would be the softest of the three (and would be in the softest range of all the mattresses they make).
Both the 5.3 lb memory foam and the 7.1 lb memory foam would be very durable materials. 4.1 lb memory foam is a "medium density/durability memory foam and would be less durable than either of the other two (although for most people that were in an average weight range it may also make a good choice because the tradeoff between durability and the feel and performance of the mattress in terms of PPP may be worth it. Once you are over about 200 lbs or so I would tend to minimize the use of 4 lb memory foam for durability reasons.
Phoenix