Hi msrozz,
The first place i would start with your mattress research is post #1 here which has all the basic information, steps, and guidelines you will need to make the most suitable choices.
A mattress is only as good as the quality of the construction and materials that are inside it and I would be cautious with the Vivon and make sure you know the density of all the layers it uses and meets the memory foam guidelines in post #10 here. They are made by a Chinese manufacturer called Zinus which also makes other brands (including Spa Sensations, Keetsa, Night Therapy, Spirit Sleep and others) which tend to use some lower quality/density memory foam.
Christeli is made by Park Place (they used to be called Luxepedic) which is a long term US manufacturer and they use good quality materials and have good value. They also have a very interesting modular design. It would allow you to just replace the topper instead of the whole mattress because the comfort layers are generally the “weak link” of a mattress and will tend to soften and break down more quickly than the deeper layers. They are also CertiPur certified.
Either way though … I would always do some local testing so you have some good reference points as far as PPP (Posture and alignment, Pressure relief, and Personal preferences) before considering the higher risk of an online purchase where you can’t test the mattress before you purchase it (and I would also keep in mind that other people’s experience on a mattress has little to do with how it may feel for you because of differences in body type, sleeping style, and individual preferences).
Phoenix