I would go for the Cosatto over this, because it's fixed using top tether and isofix, which means it's likely to perform better than a seat with the seatbelt path from the MC Nomad. There are a few videos on youtube of people fitting the Nomad and showing it's quite wobbly - what is being shown here is called forward anchorage where the seatbelt attachment point is further forward in the car than the belt path through the seat, so it can't pull tight as the seat is just sort of loose behind the taut belt. The belt path is extremely slim on this seat, so it will get forward anchorage in a lot of cars, which is no good for something you're meant to use in multiple vehicles. OTOH, top tether + isofix is an extremely universal fitting method - as long as the car has tether points (and all new cars in EU from the last 13 years do + many from the 7-10 years before this) it will fit securely.
I don't know how well the Cosatto does in crash testing. There is also a similar alternative by Kinderkraft called Fix2Go. Neither are especially highly rated brands, in independent testing, they have had some seats which did better and some which did worse. But I would choose either of these over a belt fitted version. I probably would have bought one for my own DS this summer if he hadn't been over 105cm. Particularly for short, irregular journeys it would be a good compromise to me. DO make sure that you ALWAYS use the top tether. These seats have a top tether alarm, which in the legal standard is only required when a seat fails a crash test with looser criteria when fitted without top tether. Most standard top tether seats don't have an alarm. For that reason, I would assume that the performance of these seats in general is also not especially high - but it will definitely be better than nothing, better than sitting on a lap, better than a seatbelt, better than a booster seat. Combine that with the portability and I think they are excellent - they are probably the best option that's ever been around for this age group in terms of portable seats which aren't either £400+ or incredibly heavy.
Maxi Cosi also have an isofix/TT version called Nomad XL - but it is twice the price of the Cosatto/Kinderkraft seat, and TBH I can't see anything different about the MC seat which actually justifies the higher price tag. The Cosatto and Kinderkraft also do not specify a weight limit when the child is under 105cm, which is a benefit. Maxi Cosi stick to 18kg on both versions of the Nomad.
If you can hang on to buy until the end of the month, you might get some info from the latest ADAC/Which test results, I don't know whether they have tested any of the folding models or not, but they might have. Certainly none of them failed catastrophically if they did, as they have already done a press release on the 2 models which did fail.
The other option is to get something like Halfords Impala, Graco Extend, Joie Stages etc which is RF but belt fitted. They are not as compact as these folding seats, but airlines usually let you take a child's car seat for free, you can protect it using the box that you buy it in, and depending how far you need to travel for the hire car you may be able to use a luggage trolley.