I would recommend a spray on over a drop in. I took the drop in out of my truck with the intention of doing the spray on myself. I have all the material just dont have the time. Ziebart does a great job doing it.
As for voiding your paint warranty. That is false. Thats a sales pitch to get you to buy a drop in since the company doesnt do spray on. Do you think a dealership will warranty the paint underneath a drop in when you pull it out 2 years later? No they wont. Will they warranty the paint underneath the spary on? Well whats the point, its not going anywhere now. But the rest of the vehicle paint is still warrantied.
Closing information. I used to work at a large dealership bodyshop doing combo work before owning my own business now. We did all sort of warranty work on truck with spray on bed liners.