Hi navigates,
As UTME83 suggested … the Karissa Plush Euro Pillowtop has a pillowtop and thicker layers of foam in the comfort layers and would be softer than the Mary Luxury Plush Tight Top. Which one would be “best for you” (regardless of whether it would be “better” for anyone else) would depend on the results of careful and objective testing that would tell you which one was the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
Having said that … neither of these mattresses would be a very good quality/value choice and neither one provides any information about the quality of the foam materials inside them which would make both of them a very risky purchase because most of the major manufacturers use lower quality materials in the upper layers of the mattress (above the springs) which would be the weak link of the mattress. Lower quality materials in the upper layers of a mattress (especially in pillowtops) will be the first to soften and break down (long before the innerspring) which can lead to the loss of comfort and support and the need to replace the mattress much too soon … and isn’t covered by a warranty.
Before you buy a mattress I would strongly encourage you to “reset” how you are looking for a mattress and read the tutorial post here which has all the basic information, steps, and guidelines that can help you make much better choices … and learn how to avoid the worst ones (including both of the mattresses you mentioned).
Phoenix