I've used both carrentalsavers and Costco many times to book my car. Looking at my dates at the end of August,
Budget is the cheapest from Costco. Costco is a $18 less for my 11 day trip than carrentalsavers for Budget. Costco also gives you a free second driver if you need that option.
One advantage carrentalsaver will give you is more rental car companies to choose from. I often see AAdvantage for less on carrentalsavers than anything I can get on Costco.
Both Costco and carrentalsavers are legit and there are no hidden fees. I've rented cars at MCO 3 or 4 times a year for the past 15 years and I've never paid more than the quote given to me by carrentalsavers or Costco so nothing to worry about there.
When I'm looking for a rental car I look at both carrentalsavers and Costco then book with whoever has the better price. I've never had issues renting from the 4 Costco companies (
Alamo, Budget,
Enterprise, and
Avis). However, I have had issues with some of the companies carrentalsavers quotes. In particular Fox and SIXT are companies I've had issues with so I tend to ignore quotes from those two companies.
In the past 2 years I've gotten the best price from Costco on 6 of my last 7 rentals. However, I wouldn't hesitate to book through carrentalsavers if they have a better prices. I always rent from a company at the airport but if you have Magical Express transportation to Disney World you may be able to rent for less at the Dolphin hotel.
My advice is to book the best price you can find and check prices often all the way up to when you travel. On my August trip we are renting for 11 days and there was a day 3 months ago when the price for Budget dropped from $360 to $171. I cancelled my $360 reservation and re-booked at $171. That lower price was only available for 1 day before the price jumped back up to close to $400. It pays to check back often.
Hope this helps.