Sorry nobody has gotten back to you on this one! You're right, it is complicated, but check out the FAQ on the Greenbeans site and the DIY section on TNN. The itti bitti has a bamboo insert.
I'll do my best but may get some details wrong - anyone feel free to correct me!
Terry is like towelling on one side and is smooth on the other. Fleece is the same, only the towelling loops have been brushed so they're broken and fluffy. Without being able to see the 'other' side of an itti insert it's hard to tell which it is. Velour is thinner, velvety and fluffy on both sides. Sherpa is thick and heavy, so is really absorbent but takes a while to dry - as far as I can tell most people use that for bibs or wipes.
Hemp is great but you need to prewash it (by itself) loads of times to get rid of some of the natural oils before it will be absorbent. It lasts well and only gets more absorbent, but can stiffen up over time.
Bamboo is great too but doesn't last as long as hemp. The 100% bamboo stuff breaks down faster so if you get one of the types with a little cotton in it that will mean it wears better.
When you say inserts do you mean for a pocket or as boosters for fitteds/AIOs? For pockets at night I like to use two trifolds folded together, one that's hemp on one side and microfibre on the other, one that's a double layer of bamboo velour. So when folded that's 6 layers of bamboo folded together with 3 layers each of hemp fleece and microfibre. (I understand that's on the light side though, you may need more layers than that.) The bamboo and hemp give you the long term absorbency, the microfibre soaks up the pee really fast IYGWIM.
HTH