So I own a Session 5 but it lives on my cinewhoop which leaves my 5" with a lack of a decent 4k camera. The Caddx EOS it has at the moment is trash so i thought why not just replace this with one of the following:

Hawkeye Firefly Split 4K EDIT: Read this gives bad latency back to goggles hence why the two below have dual lens I guess
Caddx Tarsier - 4K
RunCam Hybrid 4k

Two of these have dual lens setups whereas the firefly just one. All three have separate boards for onboard DVR recording.

Does anyone have any experience with these to recommend or maybe something else. They all seem to be in the £100 arena which i think is pretty good considering a decent direct replacement FPV camera such as a Foxeer Predator 5 is £30 so i'm basically getting a 4k camera for recording capabilities for £70 and saving weight. 

Where's the catch?

I have a few runcam hybrid and tarsier setups.

Runcam hybrid is much taller than tarsier. In a standard 5" frame they both generally fit but the camera mounting screws might not line up well with the holes for a normal mini cam so you need to drill new holes or use those 3d printed mounts that attach to the standoffs. If you want any significant camera angle you might be limited by the top plate, runcam hybrid especially. I just cut a bit off the top plate to give some clearance for tilt.

Tarsier has better fpv feed especially handling light changes. hybrid uses the racer camera so it has high contrast and colours are all way brighter, makes it easier to see things but doesnt handle light changes as well as tarsier. Both are really good fpv cameras though.

Hybrid has the better 4k camera. I've broke quite a few hybrids and less tarsier, this might be the size difference. Hybrid definetely does not like water or melted snow the board will fry immediately. Hybrid has a single board, tarsier has a double board. Both are 2mm stack screws, I tried drilling out a hybrid to 3mm that was the end of that board so don't think about that.

Gopro video is much much better than either of these. If your looking to do any colour grading, editing, YouTube videos these are not the greatest for this. Settings are limited you can't lock exposure and use a filter. If you want to just fly and have a camera available at the flick of a switch just so you don't miss that one off awesome day your having to show your friends then these are perfect.

There's frame designed for these camera's. They are the best choice to keep things protected. I prefer the hybrid when it's protected or tarsier for canopy style builds. Ummagawd acrobat 4k is a really good frame or the banggood frog style frame for 9x9 motors. I haven't broke a camera in either of these frame's.

That's a really helpful response so thanks. I like to colour grade my cine footage more than the 5" which is why its ended up staying this quad more than anything else. I find the higher up faster flying looked ok just on standard settings etc. I'll probably end up going with the Hybrid one as its lighter too without the WIFI board etc. I quite like the idea of the QR code setup thing too, pretty cool idea. I already have some TPS mounts with three holes on each side so think it should be ok and i'll just VHB the board down between the camera and the FC stack.

How have you found the hybrid for jellow?

No issue's with jello at all. Their heavier being a double camera for mounting with 2 screws but I never had any issues, I've read some complaints about this.
I like the hybrid, Id tried to switch all my quads to this cam but I found its lot more fragile than a regular camera. The 3" quads with a frame designed for it is really the way to go. They poke out of the front too far in a standard frame and get destroyed faster than a normal camera. Board shorts really easily too, take care to not short it on anything or wet it and their solid i found. The footage isn't bad at all but it's a bit blown out and without much control other than fps and resolution it's just not as edit friendly. As is for showing off is pretty cool though.

I've been waiting for the next hybrid version that I've heard is in the works. Hopefully it's a bit stronger.

I've just ordered one so I'll see how it goes. I'm adding the iflight GPS module too so i can try out GPS rescue if I'm brave enough to test on a switch ?

I thought I'd add this so you don't damage your FC like I did. These draw 1A and can be too much for the FC bec. Depending on your vtx you might be able to move it to vcc to reduce the total amp draw. Just be sure to review the specs if you power from the stack or you may fry it without an external bec to power this.