I think boss is the only polly v-plow maker. There isn't much of a weight difference between polly and steel on light duty trucks (some times polly weighs more), highway blades the weight savings is there.
Yes you don't have to worry about snow sticking, or repainting with polly. However with the new paint technologies being used by all manufactures, snow doesn't stick often, and scrathes rarely occur.
As for the spreader. The two stage fisher speedcaster 2/ western pro flow 2, is the only type of tailgate i would look at. Now this is because i want to be able to spread sand, so i would need the two stage model (pro flow 2/speedcaster 2) For pure salt a single stage would probably work fine (ie pro flow 1/speed caster 1).
Now form a cost stand point, you can find tons of 8' v box spreaders for 3K or less. With these you don't have to get out of the truck and reload the spreader, just press a button and drive, till the v-box is empty. A single stage tailgate (pro flow 1/speed caster 1) is around $ 1000 ( a 1/3 of the cost of a v-box), speed caster 2/pro flow 2 ( around $ 1800.00 to $ 2000.00 ( about 2/3 the cost of a v-box). So the 2 stage tailgates are so close to the cost of a v-box, and a v-box does 10 times the work, i would go with a v-box. Now there are cheaper single stage tailgate spreaders out there, however i believe you get what you pay for. I know a guy that used a tailgate for 1 year, got so tired of loading material/ and unclogging material. He went out a bought a v-box 1/2 way through the winter. Now he lost tons of money selling the tailgate, so in the end it would have been cheaper to start with a v-box. Buy a v-box, and buy right from the start, so you don't out grow your spreader.
If you are worried about removing it, it isn't that hard to do. Just build an over sized swing set, and get a cheap chain fall, i know with that set up you can remove/install the v-box in under 15 mins, the tailgate isn't much faster.
Geoff