the Helix Sleep mattress has 2" of bouncy foam, 2.4" of springs and 6" of support foam. The problem is that the spring layer feels like little metal springs, so you're effectively sleeping on 2" of bouncy foam. This mattress is customizable, so it's possible it would be better in other configurations.
My girlfriend had similar problems with the Leesa, which has 2" of soft bouncy foam and 2" of memory foam. I thought it was comfortable, but it hurt her hips when she slept on her side. The mattress she eventually picked has 4" of latex in the top two layers, and I suspect it works better for her because they are more supportive (although still very soft feeling).
This is the kind of mattress most people are familiar with, and probably what most people will want. If you're familiar with a spring mattress, imagine that but with the ultimate pillow top where you can't feel the springs at all.
If you want this kind of mattress, my favorite is the Leesa, which has 2" of bouncy (non-latex) foam followed by 2" of memory foam (technically a hybrid; see below). My girlfriend prefers the "#BestMattressEver" from Brooklyn Bedding, which has 4" latex in the top two layers. Be warned her "#BestMattressEver" full mattress was slightly smaller than any of the other fulls we tried.
You can also find extremely cheap mattresses on Amazon with 3" of latex in the top layer(s), but I have no experience with them.
This type of mattress initally feels hard, but quickly conforms to your body as it warms up. Some people describe it as awful quicksand-feeling, and some people think it's the greatest material of all time. Memory foam tends to be hotter than springs or latex, and the "cooling gel" memory foams are a joke. If you don't love the feel of memory foam, you'll probably be happier with a bouncy foam.
Memory foam is measured by density (the weight of one cubic foot of foam), and there's a complicated relationship between foam density and firmness. Higher density memory foams tend to be higher quality and more firm. Because of how memory foam works, "more firm" doesn't necessarily mean "hard" though, since a higher density memory foam will still conform to your body. Tempur-pedic uses 5 lb memory foam, and in my experience, it makes a big difference. 3 lb memory foams tend to be too soft, and don't contour as well.
If you want this kind of mattress, I recommend buying your mattress from Costco. They sometimes sell Tempur-pedics for around $1500, so the $50 membership is nothing compared to the $2000 price difference. They didn't have any Tempur-pedics in stock when I bought my mattress, so I got a Sleep Science Ara (also Costco-only), which was $600 cheaper and has excellent materials (6.6" of 5 lb memory foam in the top two layers).
I haven't slept on it yet, but my parents have the eLuxurySupply mattress in their guest bedroom, and my initial impression was that it's surprisingly-decent for a $450-650 mattress. I'm skeptical because it's only 3" of 3 lb memory foam, but my girlfriend's (one night) experience was that it was surprisingly comfortable for her, although she's not a fan of the memory foam feel.
If this is your first memory foam mattress, you should buy from a company that allows returns, since they're expensive and most people don't like them. Be aware that mattress stores frequently only offer exchanges, and some companies (like Tempur-pedic) won't refund shipping costs.
You can buy a mattress with a combination of memory foam, bouncy foam and/or springs. The top layer is really the only thing you'll feel, but these can significantly effect how hot the mattress feels.
The Helix Sleep mattress is a bouncy foam/spring hybrid. It sleeps cool and the top layer is very nice, but I thought the springs were uncomfortable. It's customizable to a certain extent, so it's hard to say if my experience is common.
The Leesa is an bouncy foam/memory foam hybrid, but I'm not sure what the point of the memory foam is. It feels similar to a pure latex mattress.
If you like the feel of extremely hard mattresses (sleeping-on-the-floor hard), Tuft and Needle has a well-made, very hard foam mattress. This is not memory foam so it won't conform to your body. I used this as my primary mattress for a while, and while it was a huge step up from my ancient spring mattress, it was way too hard for me.
Most mattress warranties require that you use a mattress protector, and keeping your mattress in good condition will allow you sell it for more on Craiglist if you decide to get rid of it. One thing that surprised us was that different mattress protectors significantly altered the feel of our mattresses.
Unfortunately, most reviews of mattress protectors are comparing them to either nothing, or a mattress topper. Very few people compare mattress protectors to each other. I've previously used random $30 mattress protectors from Amazon (like Protect-A-Bed) and they all feel the same and made my mattresses feel a lot worse. This time, I took a chance on a Malouf Sleep Tite mattress protector and I can't recommend it enough. It's so thin it's translucent, and it when it's on a mattress, it feels like it's not there at all. Don't buy it directly from Malouf though, since Amazon sells them for a lot cheaper.
Toppers are awful and I don't recommend any of them. Buy your bed from a company that allows returns instead. The only time a topper would be good is if you have a well-made non-sinking mattress which is way too hard (like a Tuft and Needle), and you can't return it or buy a new mattress. In that situation, I would buy the Serta 4" 4 lb memory foam topper or a 3" or thicker latex topper. Tempur-pedic also makes a 3" topper which is probably good, but it's $300, I've never tried it, and they don't accept returns.
Follow this flowchart (edit: made much nicer by Kiley Graim!), and if you run into problems: