I've used both many times over the last ten years. I consider them interchangeable. For a run to a Downtown hotel from LAX I'd go for the one that has the lowest rate (although I would doubt the difference would be much, but both are way cheaper then a taxi). They both seem to have a ton of vans always cruising LAX.

If you are a member of AAA, one or both of them regularly offer discounts. In fairness I've been p*ssed at both of them multiple times. Usual reasons: longish waits at the curb, multiple laps of the airport, etc.

One final note about their rates. Prices to a specific address such as an office building, might be more then to a hotel. So if you are heading to an office building, try to find a hotel right near it, and have the vans take you there (and walk to your office, chances are there is a hotel within a quarter mile of any office building Downtown).