We rent a car when we fly into LGB. The car rentals are directly across the street...literally about 50 feet away. It is super easy.
I have found the past 2 trips that Alamo is the cheapest in Long Beach. I always use Costco Travel (their online tool is easy to navigate). This trip (we leave this coming Thursday - woo hoo!

) our total price for 6 days rental with tax and everything for a full sized car is $144. There are 4 of us, and I think that is pretty reasonable. We would spend close to that for a shuttle, but with a car we can go to the grocery store and the Disney outlet too (which we like to do at least once a trip).
Good luck deciding what is best for you.