Hi dian,
If you are looking at online choices that offer among the best knowledge, service, quality and value in the country … I would suggest looking at the choices listed in post #21 here.
A forum search (the button that says “forum search” not the one that says “search main site”) on any of the businesses you are looking at will bring up lots more information about all of them and some thoughts about their relative value compared to other online choices.
Having said that … it is always preferable to look locally before you expand your search to online sources. This can help you find the best local value (which may be very comparable to online sources) and it will also give you some personal experience with different combinations of materials no matter where you end up buying a mattress.
The better local choices in the NYC area is in post #2 here and there is a more categorized list with more detailed descriptions of many of them in post #7 here.
Hope this helps
Phoenix