I'm assuming they have the 2 in the name first because of oxygen, and second because there are lots of products already named Optima and O2ptima avoids any copyright issues. I leave out the 2 most of the time unless I'm writing Dive Rite, as I was scolded once
Bumping your head isn't really any issue at all, the stand and harness are adjustable. You bump your head on doubles manifold, but it's never been an issue on the optima for me.
I've never seen a Prism 2 in the wild.
Their can is "engineered thermoplastic" and it's clear. In theory that lets you see a flood. In practice, your buddy is usually far enough away that they don't see your can, positive and negative checks should avoid leaks, and if your loop got a tear you would hear and feel gurgling. Optima has a Delrin can.
Prism head has 2 barrel orings and clamps on with 3 latches. Has a separate sorb canister that drops into the can. Radial scrubber. Optima has a face seal and a barrel type seal, and the can has an attachment on both sides. The can and sorb holder are one and the same, and the head goes on one side, water trap on the other, and it's a bayonet connection. Axial scrubber. In theory the dive rite looks like it could break down into smaller parts for traveling, but in practice I don't think it's a huge difference, and "traveling" means a lot of different things depending on if you mean driving, flying domestic, or flying international.
Prism claims to have the lowest work of breathing.
Prism DSV inside is stainless steel. DSV uses a threaded connection and it has one mushroom in the DSV and one in the hose. HUD mount clamps onto one of the threaded sides and it looks like it has a little more adjustment range and appears to be molded plastic. Dive Rite DSV is Delrin inside and out. Uses a bayonet clamp with both mushrooms in the DSV. Hud clamp is on the hose and it's 3d printed ABS. Never had an issue with the adjustment range.
Prism head has 2 9v for the solenoid and 1 SAFT cell for HUD, built into top of head but held on with latches unlike the JJ screwed on cover.
Prism uses 3 cells and has a one piece harness, large molex on one side splitting to 3 molex. Dive Rite uses 4 sensors and has 4 harnesses, molex on one end and banana plugs on the other end. In theory you could clean the banana jacks on the head more easily, and if a harness was damaged you could replace just one. The harness is not cheap, though, I think $100, and it's an annual service item. I imagine the Prism molex cables are cheaper to replace, plus if you only have 3 cells then you have saved another $75 or so per year.
The Prism just looks to me like an open water rebreather. The thing I dislike most about the Prism from researching them is the mushroom valve in the hose, the hud clamp, and the 2 9v for the solenoid. I am always careful with mushroom valves and it's easier to just have one part to be careful with than 2; I don't want to have to remove the hud clamp to remove DSV, and I am fine with just one 9v, but I only use the lithium ones.
But it does look like a decent rebreather and it's worth doing a pool dive on it to see if it feels right for you. If you have a good local community and local support options, that can get rid of the issue of less than ideal customer support.