Nelson
-Can only use the Trip 16" and Quad Ds with sufficient ammo.
-Great for rushing, especially if they're blind
-No scout
-Pretty good PHH capabilities
-Gun angles are decent


Valiant
-Is a skunk
-Can use Hood guns but better
-A bit slow in some cases
-Gun angles aren't the best
-Ship size is great
-Can't rush well
-10 Tslots 

I think the biggest difference is that, Nelson can only be operated by Nelson BO but Valiant can be operated by any sailor.