There's 2 main types. Liquid and powder.
Powder requires you to make up a solution beforehand - usually in quite a large quantity, by mixing the powder in water. XTOL is I think 5L so you'll need somewhere to store it. Once made up, powdered chems don't last as long (a few months). They're usually quite cheap and often you keep the solution after you've used it and "replenish" it every so often rather than chuck it. It works well for labs who are developing a lot of film. For me - as a home developer, I bought some XTOL and found it a massive faff. 120 film usually requires 500ml of solution. 35mm usually needs about 300ml (it tells you on the tank how much you need) so 5l goes quite a long way. Not quite sure how many re-uses you get out of it either.
Liquid developer is usually mixed with water at different concentration levels, then you throw it away when you're done. I found this much more user friendly. The 3 developers I've tried are Ilfosol, Rodinal and HC-110.
When you're looking at developers, it's great to try something wacky and out there, but it's often difficult to find developing times. So some people try different dilutions to gfet different results, and it's part of the fun of developing your own - you can experiment! But if you want standard results and don't want to mess about, using recommended dilutions is the way forward.
Rodinal typical dilution is 1+25 (1 part developer, 25 parts water). HC-100 has two common dilutions: A (1+15 I think?) and B (1+31). Ilfosol is 1+9 so you can see you go through a lot more developer per film with Ilfosol than you do with Rodinal or HC-110. Ilfotec-DDX is very highly regarded, but it's a 1+4 dilution and is thus significantly more expensive than the others. Here's my (very sad) chart... Not sure how in-date it is now though. It was a while ago I did it.
For me, I found Rodinal great for high ASA clean films, and with high grain films it was pretty bad. HP5 in Rodinal is awful (in my opinion!), but TMAX100, or Delta 100 are lovely. This year, I've swapped to HC-110 and found a staggering improvement in my higher speed films with no noticeable difference in teh low speed stuff. TMAX100 still looks great, but now, so does HP5. If you shoot predominantly 120 though, you won't notice it as much because the grain is likely much less pronounced when you scan.
Finally, the Massive Dev app is brill. It allows you to store your times and films, and it tells you how much developer/water you need to mix depending on your concentration, and it also has a timer. It's £7 or thereabouts, but it's the best money I spent.
I'm sure others will chime in, but that's my tuppence.
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