I doubt there is much in it TBH - if you can see the Axkid lady near you then you could try those out and see if you like them? If they aren't what you want then look further afield but honestly the biggest difference is between ERF seats up to this size vs the smaller isofix/spinning options. The differences between the ERF models themselves are fairly small. There are some but they tend to be personal preference and I think you can get hung up on tiny differences online. Focus first on what you can physically see if you can see any at all. You may also want to join one of the ERF/UK car seat groups on FB and put a post out with your area and ask if anyone is willing to let you come and see their seat. Or if you have a local mums group/network you could ask in to see if anyone has an ERF seat you could look at.
In terms of height differences, the old 25kg seats with no height limit last up to approx 125cm. The 36kg ones have a 125cm height limit but most of them have quite a long backrest. I've seen both Minikid 4 and Klippan Opti 129 with the Q10 dummy sat in them which is 138cm tall. Technically they are out of regulation once the child goes over the 125cm, so not recommended but the physical possibility is there e.g. in case your child has a long body.
The only one of notable difference is the Avionaut Sky (which is not 36kg, anyway) which has a shorter backrest despite being a 125cm limit and lasts about as long physically for height as a Joie seat which is about 110-115cm. And the isofix 125cm seats have the 23kg weight limit, so watch those because that could be hit before age 4 in a 99.8th centile boy.
Margaret from Rear Facing Toddlers has a good video on Youtube comparing the backrest heights of all the 36kg seats plus an older 25kg seat and the Avionaut. And she has some photo albums on her FB showing older seats compared to the "up to age 4" seats.
But for the vast majority of children, which includes 99th centile boys, any ERF (125cm/25kg+) seat is fine, and will get you past age 5, likely close to 6. If you have a child well over the 99.8th centile or a child with low muscle tone, then these small differences do start to matter more. But for most people it doesn't matter.