well to start off with walrein's stats are much higher and he is better as a Special Attacker.
Now Dewgong has a deeper movepool and you can use him as an attacker or Special Attacker, but the best moves on them are pretty much the same. But I would go with Walrein on this one, he is stronger in every stat and movepool is a bit different but the good moves are pretty much the same
Ability: They are both good, depends on what you want.
Item: Nevermeltice or Icicle plate
if you want a Dewgong (but I said Walrein is better)
item: Icicle plate or Nevermeltice
Ice Shard/ Icy Wind/Aqua Jet
All in all, I say Walrein is better, I have used him before and he served quite well.
Just mix and match to what suits you
After all it's your choice,
Hope I helped :)