If surface was prep'd/treated properly (degreased/metal prep) POR-15 would be more effective at stopping and preventing rust. It's a rust encapsulator. POR-15 takes an added top coat to protect it from UV rays. Bedliner is effective at preventing nicks from stone damage and since it is thick - it cuts down on road noise but it is heavy.
I used rubberized undercoating on a then new 1972 Porsche 914. It ended up trapping water and probably causing more rust than it prevented (I lived in NJ and No. VA). The rubberized coating separated from the metal and caused air pockets. In hind site, I did no prep other than ensure it was clean of mud/dirt. Hopefully bedliner doesn't do the same - guess prep (de-greasing) is key.
Bedliner or rubberized under coating would be good in wheel wells where stones get thrown up as it would prevent spider cracks of fiberglass and also deaden sound (but I doubt the sound deadening would be noticeable).
The bed liner sure looks nice though!
It is all in the prep...
Prior to laying the bed liner down, the surface was completely sanded (using different grits). I removed everything (to almost bare metal in some spots). Next, I cleaned the service and degreased everything with a heavy cleaning solution. Lastly, I painted the service using a rest prevention paint. After the paint cured, I applied the bed liner material.
In essence, I created a rust proof and sound deadened chassis with added weight.
What made this time so much easier then last, I have a 30 gallon compressor and air tools. It made the process a BREEZE compared to hand sanding and painting...