(Disclaimer - have not owned either seat - you will struggle to find anyone who has owned both. I used to work with car seats so have some hands on experience.)
Main difference is that Cloud T is much longer in the shell, so although both have the same 87cm height limit, you won't get anywhere close to it in the Maxi Cosi.
Maxi Cosi does allow for the flatter position to be used when the seat is belt fitted, whereas the Cybex doesn't -you can only use this on the base or on the pram.
The slide is pretty incidental. If you plan on getting the toddler seat that goes on the same base, I would recommend testing for fit in your car when you test the infant seat. (99% of the time, it's not worth getting the toddler seat for the same base, anyway).
Both sets with the rotating base are pricey, around £450. It is worth considering whether a basic model such as Maxi Cosi Cabriofix i-size or Cybex Aton B or Britax Babysafe Core (which cost around £200 for seat + base) would meet your needs and whether it is really worth the extra £250. There are also some in between of course. But a lot of the time, the marketing is heavy on these premium models so people often end up there as a starting point. IMO, it makes sense to start with the most basic model, and look at how much extra you'd pay to add certain features (by upgrading to the next model) and then consider individually if it's even worth it.
In a lot of cases, you're best off saving money on the first seat in order to invest into the second stage seat, because you'll use it for longer and the quality/safety is much more variable.
If you are keen on the premium sets, it's worth looking at the Besafe Beyond system, because the second stage seat for that one is currently the only rotating seat up to 125cm (all the others go up to 105cm only).
The pram compatability for Maxi Cosi, Cybex, Britax and Besafe is the same. Nuna, Joie, Avionaut and some others also go on the same adapters.