Hi OP
There are 3 comparable seats on the market currently. These all have a 157 degree "lie flat" position which can be used in the car and the lie-flat position in car is against the direction of travel, NOT lateral as most previous carrycot car systems were previously. This is likely to be a big improvement for safety.
Nuna Arra
Joie i-Level
Maxi Cosi Marble.
You cannot fit any of them with a seatbelt, they only fit on the isofix base they come with.
The Maxi Cosi is brand new and has only just come out so there's hardly any info around on it.
Joie and Nuna are the same parent company; it's possible they are actually very similar in design of seat with different fabrics/inserts.
All brands get high scores in general for their infant carriers for safety.
Only the Joie has been through independent testing (Which?)
You need to subscribe to Which? to see the full review and test scores, but as they do their testing in conjunction with other European consumer publications, you can access the results online from other countries. ÖAMTC which is the Austrian version publish some of them. The Joie i-Level gets a good score for side impacts, but the frontal impact score is the most interesting as this where you'd expect a lie flat seat to show weaknesses (we know for example that for seats for older children which are rear facing, the more reclined they are the worse they perform, in general.) It gets a 1.9, which is overall good, but lower than most purely rear facing seats which score between 1.2-1.7. Lower numbers are better and the scale goes from 0.5-5.5, so to convert to a percentage, the Joie i-Level gets a 72% score in frontal impacts, compared to the 76-86% that most rear facing seats get. It's likely that when it's in the fully upright mode, it performs better and may be within this 76-86% range. I think I'd recommend using the lie flat mode in the car only if the baby is already asleep and/or it's a long journey and/or if they dislike the upright mode.
The Joie is the only one of the three that you can get a toddler seat which goes on the same base later, if you'd like to do that. However some people say that the headrest is too cramped and squashes their baby's head and they don't like this. This doesn't seem to be a problem for other people, so it probably depends on the baby.
It is also worth noting that as there's no separate research on lie flat car seats, they aren't exempt from the usual guidance about how long to keep babies in them at one stretch (up to 30 mins up to 4 weeks / up to 2 hours thereafter). Of course you can use your own judgement and make a decision.
Hope that helps :)