Hi Cooperman,
Both of these use good quality materials but are also quite different.
The Ultimate Dreams Eurotop is a latex / polyfoam hybrid which uses 3" of Talalay in the top layer and has a base layer of high quality polyfoam and has a polyfoam quilting layer with a bamboo cover. One of the main advantages of this mattress is that you can choose the ILD (softness level) of the 3" latex comfort layer so you can customize it to your own personal needs and preferences and can exchange it for a different firmness choice if you need to at a reasonable cost. In the king size you can also spit the top layer to have it softer on one side than the other for couples who have different needs and preferences. This design (with a customizable and exchangeable top layer) is similar in concept to some of the mattresses that they sell on their own Brooklyn Bedding site except the layer exchange options for the mattresses on their own site is more liberal and has no cost involved.
The Ecosleep Cassidy is an all latex mattress which uses higher quality materials in the deeper layers and in some places also has very good value but there are no options to choose your comfort level or customize it after a purchase. If you have tested it in person it could make a good choice if you know that it is suitable for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). If you would be buying it online without testing it first then your confidence in its suitability would be a significant part of its “value” because it has no ability to customize it or make changes after a purchase and no matter what the quality or value of a mattress … it would have little value to you if it wasn’t suitable for your needs and preferences.
Both of these could make good quality/value choices depending on which parts of your personal value equation are most important to you.
Phoenix