TBH I think they are both good options. You may also want to look at the Britax i-Sense system which is very similar to the other two and has also recently come out.
The Cybex system has been out longer than the other two so you'll find more reviews etc about it.
The Maxi Cosi doesn't have a lie flat feature, but with the newborn inserts it is almost a completely flat surface for the baby, which is nice. None of these car seats are lie flat enough to replace a carrycot, but the recline on the Cybex is nice for things like shopping, doctors, or bringing the baby into the house to finish a nap. Britax has a sliding system inside the seat itself which means when it is adjusted to fit newborns it's flatter, and once you adjust it to fit older babies it's shaped more like a reclined chair.
They both have a nice high limit so will last you a decent amount of time. I believe the Maxi Cosi is a bit lighter to carry around than the other two. Maxi Cosi also have another option for their spin base called Coral 360. The Coral consists of a hard shell for when you want to use the seat in the car or on a pushchair, but a soft inner carry portion which you can take out of the shell and carry into the house/doctors/etc - this actually seems like a better use of the spinning base for infant carriers to me.
All 3 go on the same pushchair adapters (Maxi Cosi/Cybex/Universal type, not Britax type, those are for older Britax seats), so will fit all the same models. Cybex is probably "officially" approved for more simply because it's been out longer, but you don't need the official approval especially.
All 3 toddler seats are good - but only last until 105cm so not the best if you end up with a taller toddler unless you have a second baby to hand down to. I think Britax is my favourite of the spinning toddler seat range. I find Cybex clunky (although it works fine) and Maxi Cosi have been getting odd safety results for their toddler seats recently - they say this is to do with the way the Which? (etc) tests look at side impact crashes which don't suit their seats. But their other Pearl models have always done really well.
Worth bearing in mind about spinning infant seats in general:
You're paying a lot for the spin feature, which is not realistically that necessary. Infant carriers are removable so in practice you tend to get the baby into the seat in the house, so they can face you easily anyway, and then just click it into the base. Until you get to the next stage of leaving the seat in the car, the spin is sort of redundant. It's very nice to play with in a shop and it looks snazzy but it doesn't really add any functionality you didn't already have - and if you do plan to leave the seat in the car all the time, then you may as well go for a fixed 0-4 years seat rather than an infant carrier. The situation I think it's suited for is if you want to leave the seat in the car 90% of the time but have the option to take it out occasionally. But I think you pay a lot for this and so I'd question if it's really worth it.
Now, I said rotation is useful for the next stage up and I stand by that, it really is. But if you look at these modular systems (infant seat, base, toddler seat all compatible) from a bigger picture perspective you will often find that they are fantastic from the manufacturers' point of view - they have you captive for the next seat up already. But they are not always practical from the consumer's point of view. Reasons being:
- They cost a lot. You often pay £100+ extra for the base compared with one that only takes an infant carrier seat. Next-stage seats, 90% of them don't need a base. They fit directly to the car with the seatbelt (and then you don't move it) or they have the isofix base built in. You would think the modular seats would be cheaper than the "no-base" seats but they are not really - maybe about £30-50 cheaper - sometimes they are more expensive!
- If you have a second baby, your infant seat base is in use for the toddler seat. That's not very practical, because you either have to buy another set of bases for each car (At twice the price of a baby seat only base!) or use the baby seat baseless, which means you lose some of the functionality of it like being able to spin it, which you've paid £££ for. Or you can move your toddler to a larger seat which doesn't need a base, and save the toddler seat for your younger child when they outgrow the base. But that's then more money in a short space of time.
- There are plenty of toddler seats on the market which rotate and extended rear face. You don't need to get one as part of a system.
- All these modular seats are limited to 105cm for the toddler part. That's OK if you have an average or smaller sized child, if you don't mind moving to a booster seat aged 3, or if you plan to hand the seat down to a younger sibling. But if you were hoping this system will get your child to booster age and you're not comfortable with them being in an adult seatbelt until more like 4 or 5, then you may end up needing to buy another larger seat later and feel annoyed you didn't just get the bigger one (none of which work with a base) in the first place.
I'm not saying that you shouldn't look at spinning infant seats. They may well be the best model for your family, budget, circumstances. Sometimes other features of the seat (like the recline on the Cybex) are the seling point anyway. But I do think they are overhyped, and it's worth being aware of the full picture and costing up alternative options all the way through to age 4.
Also with the price of a lot of the modular systems, it's worth looking at the Recaro seats Zero.1 elite / Salia elite. Not hugely popular in the UK, I think due to the price. But they are actually cheaper than a full set of these modular seats and have a lot of the same benefits, so I think if you're looking at them then it's worth giving the Recaro seats a look too.
One last tip would be to find a stockist of the Maxi Cosi seat (since it's newer) and try both the infant and toddler seat out in your car to see how they fit. And the Cybex one. (And any others you want to look at.) There is no use buying a modular system and then you go to move to the toddler seat and find it won't fit behind the front seat. See how easy it is to rotate the seat(s) in your car, see how they fit, see how easy it is to get the infant seat on and off the base and simply look at what you prefer.