The most significant change in mattresses on the market now is that they won't fit easily into older bedframes. For several decades the "standard" mattress and box spring were each about 6 to 8 inches thick, giving you about 12 to 16 inches or so on top of the slats in your frame. The "sleeping surface" in typical frames would be about 20 to 26 inches above the floor, depending on how high your slats were.
When we bought our most recent replacement, about 2 years ago, we were unable to find either, base or mattress, less than 9 inches thick, and some "super" versions were up to 14" thick. The difference is due to very thick padding, on the top surface of the supporting "box spring" and on both sides of the mattress. This puts the "new" sleeping surface 18 to 28 inches "above the slats" if you put them into an older bedrame. This can give you a sleeping surface as much as 35 inches above the floor, or roughly like sleeping on the dining room table (or on your pool table if you're recreationally inclined).
If you like the "fitted" bottom sheets, you'll likely find that older ones won't wrap around the new mattresses far enough to stay in place. Newer ones intended for the thicker mattresses are available, but the packaging often doesn't indicate which are, and which aren't, suitable. Especially at the "discount" outlets, you'll still find the old style that aren't usable.
If either of you are a bit on the "short-legged" side, you may find it annoying having to jump up onto the bed at first, although you'll probably get used to it after a while. Some new frames virtually eliminate the space under the box to partially compensate for the thicker new styles.
The change in construction is largely because foam pads are cheaper than good coil assemblies. For the new styles, number of coils is less significant, and many manufacturers don't even tell you how many coils they use. You can get "all foam" styles with no coil springs. Each brand has its own "magic" foam composition, and each will tell you that theirs is "superior to all others;" but there really is no way of telling what you're getting. (Anything they tell you was written by marketing people, who's motto is "shut up, pay up, we'll sell you something different when you find out this is shit.") Often the one that's "best" when new is the one that will fall apart quickest; but there's no reasonably accessible info you can use to evaluate them in advance of "wearing one out."
Even the advice to stick to "known name brands" is suspect, since the discount brands have access to the same foam materials as the big names, and neither will tell you, in useful and meaningful terms, what they've used.