Heyha yes you've understood all that correctly :)
It's the side impact that makes the difference. You could look at Britax Dualfix 2R instead, this has better side impact protection than the Joie, but if you go for the non i-size then you don't have it limited to 105cm. I am not 100% sure how much taller the Joie is, if anything, than the Britax. I have put them next to each other but can't remember off the top of my head - sorry!
Joie Spin Safe was tested in the most recent batch of seats for ADAC/Which? and got a really good score for frontal impact, which means that the normal Joie Spin 360 will have a similarly good score, in rear facing mode. The score it gets for frontal impact in the original is likely to be in relation to forward facing mode (I'm not 100% clear on whether these are an average or the worst score is used, different sources explain it differently).
You have to remember a lot of these seats are designed around a max child height of around 105cm anyway, they aren't massively taller. For us it made sense to go for the non-i-size, because first of all we're in Germany and the version sold here (GT version - maybe see if you can find this in the UK?) does have side impact protection in it, but secondly because I specifically wanted as much height as possible from an up to 18kg seat! If you aren't concerned about outgrowing by height then the difference between an i-size one and a non i-size one is likely to be negligible and I wouldn't worry about it.
Other well rated ones both for frontal and side impacts with the option to both RF and FF:
Non i-size (up to 18kg, no stated height limit)
Britax Dualfix 2R
i-Size (limited to 105cm - may have 18kg / 19kg weight limit)
Britax Dualfix i-Size / Dualfix M i-Size
Cybex Sirona S i-Size (you also have the option of Sensorsafe)
GB Vaya 2 i-Size
Recaro Zero 1 / Salia
Nuna Rebl Plus
Besafe izi Turn