Think you've already got the salient points in the answers above.

I've stayed at the Hilton LAX a lot (it's a decent hotel), and yes, it is marginally nearer to the terminals than the Hertz facility, but whether it will be any quicker will depend on how many stops your respective shuttle bus makes. Key factor I would have thought is that the hotel charges $20 a night for self-parking (and more for valet), so if you can save that, then it is worthwhile. As others have said, it is about a 10 minute walk from Hertz back to the Hilton, so your other option is to drop the car at Hertz (and close the rental the official way) then walk back. It's fine in the early to mid evening, but yes, probably not a good idea late on.