I'm visiting the UK and am thinking of flying Emirates. Their web site gives me an option to book a car which I can use upon arrival. I'm looking for a good deal - so my question is would I get one because Emirates has better buying power than me as an individual and are they likely to pass on some of the saving to me?
Car rental is one of the most obscurely priced services with varying conditions. It is very hard to predict and my advice regarding car rental bookings is "compare". Compare multiple car rental agencies, compare with brokers, compare with the deal you can get booking with an airline or online travel agent (OTA).
The reason you are suggested this car rental is not so much that they are offering you a particularly good deal but just that the OTA - or Emirates, through their ecommerce department - is making a margin on every flight/hotel room/car rental booking (or in the case of flights on all the small extra called ancillaries, like seat reservation, upgrade, shuttle ... This margin is usually the same for all bookings on one provider (to be confirmed) and is determined by the GDS (which is the bookings wholesalers).
Specifically regarding car rentals, there are some conditions that vary depending who sells you the package, like insurances, included kilometers, driver's age ... There are a lot of fees for options like additional driver, fuel refills, one-way rentals. So overall the price depends more on these - and they also often matter to you - so that every booking price is different (I discussed that in another answer).
I just tried to book a car rental through the airline I booked a flight with recently (they apparently have a contract with a broker) and compared with a booking from the website of the same rental agency and in 2 cases I got a lower rate using the airline link. I tried with another booking with another carrier and this time it was through an OTA. In that case the booking through the rental agency was priced the same but maybe the insurances were not exactly the same (I could not see the conditions).
In the end what matters is that the price of a car rental with a specific combination of options might be different based on who sells it to you (the rental agency, an online travel agency or a broker). In my experience, the broker was usually cheaper, but I would not consider it a rule. And the conditions - especially the liability insurance - are not always clear. I make a point always reading the conditions (at least insurances and distance limits) because there might be tricks.
So if the price matters to you, get a quote with Emirates, go to the rental company website and get another quote, and go to an online travel agent like Expedia/Hotwire/... and get a third quote. Compare the prices and the conditions and pick your favourite.