I have a Manfrotto MMC3-01M monopod. I held off getting one for a couple of decades. Here is my experience.
As Derrel said, you don't use 'em for long exposures. In many ways it's not a simpler version of a tripod--it's a completely different beast. I use it to stabilize a really big lens (like a 600mm) when I'm shooting sports or birds. Additionally, some people (mostly hikers and nature photographers) like their monopod to double as a walking stick (so keep that in consideration when you buy one). And most people I talked to had been using them a while had gone relatively cheap (as opposed to buying a tripod where they were willing to pay a lot more for composite or specific traits). Why? Because they get left or lost in a lot of places (because the best monopods are small and light--so easily lost or forgotten a lot).
I'm not sure I see the value in any kind of head for a monopod. Again, the work best stabilizing a heavy camera setup so you're almost never going to be hand-held with them. Additionally, some people liked having a base (4 small feet) that attaches to the bottom of the monopod. Frankly, I'm not a fan of those. The idea is you just leave your camera and lens on top of the monopod and let go because you're not shooting or focusing. Since monopods are almost always light, that means they're going to be top-heavy and easy to blow over (even with a small set of feet on the base). So I wouldn't go for that addition.
It seems to me, you have one primary choice to make: do you get legs that extend and then tighten by a clamp or by screwing/unscrewing? And to me, that seems a matter of preference--what are you most comfortable doing with one hand (because you'll almost always screw your camera body on first and then extend the legs--easier to do it in that sequence than to extend the monopod and then try to screw a 5 foot appendage on to the bottom of your camera when it's got a huge lens on it).