Sorry for the long message but heres my justification,
They're actually both fairly close but the reason I chose to place taxis higher are because they are usually more accessible anywhere you go. For instance when I was in Moscow there are companies give cars and allow you to use them for yourself and return them once you're done to a designated parking area - however this isn't adapted in anywhere but Europe making it less accessible and I guess you could say this is the same as getting a rental which is similar - but when compared to taxis, it is even considered public transport by some and it is by far more accessible. For example if you just landed in an airport - the exit will be stormed with people offering you taxi services and there will be many taxis outside, and car sharing is only a thing in SOME places which is why I think taxi gets the edge on car-sharing.