What's more important, the panels and inverter or the install?

Well, to be fair they are both equally important, however, if we are talking about what to watch out for, the vast majority of problems come from poor installs.

Case in point this roof below. A fellow installer saw this on his walk and posted into a group. Apparently there were lots of similar jobs in the street!

As you can see, cable running all over the roof. The obviously only had staright conduit, so instead of putting it through the roof, they just stuck it all over it, without, it would seem, a care in the world.

The only excuse I can think of for this us some kind of emergency. But even still, no, not good enough.

So, if there is $500 difference between a quote, and the more expensive company seems like the safer option, take it. It's more expensive to save $500 upfront and spend another $4000 having the system fixed later. And warranties? Well if this is the kind of job they do, you want have one, no matter what your paperwork says you'll never see them again.