Don't skimp, it's about your child's safety. In my opinion the specific one is a must! Further you can check the compatibility at the website of Maxi-Cosi.

No, I do not think this one is, as this is not mentioned in the manual. I think the CabrioFix (without the i-Size mention) is the one suitable for airplane travel, as I have seen this mentioned in the manual available online. Check the manufacturer website for details, as you will find there the manuals for both products.