I routinely spray as many products as possible. The bed liner material is not one of them. To apply the coating, you only need to pour the product into a roller tray, and while using a roller with an attached handle, coat the flooring as if you were painting the floor of your house. It has a high solids content and has to be reduced to spray with most guns, other than an undercoating or fiberglass gell coating types. Spraying will involve more masking, prep and clean up time.

There are almost no fumes while rolling, a respirator would be necessary while spraying. The application time while rolling is about 15 minutes per coat, ( for a 3H trailer) about double that of spraying. Add in the clean up time, and spraying will probably take longer. Plus, instead of two coats while rolling, you would need additional spray coats for applying a thinned material, to effect an adequate thickness.

Your choice, but rolling is easy and less messy, with less material handling.  BOL

I recently did mine.. per Gard's instruction.  I used Herculiner because I could not find the duplicolor brand.  Mine went on fairly easy.  One thing about rolling is that the second and third coats are a little more difficult due to applying black liner over a black floor.  Have a method for rolling subsequent coats so as to be sure to cover it evenly.  Also, paint the floor seams and edges first.  I did the edges, but didn't think to do the seams first.  I saw scattered gleams at the seams when checking it by flashlight that night.  I went back over the seams specifically the next morning to ensure a good coat...  

Good luck! You'll be glad you did it.