Shane gave you about the best response regarding ability use.
Regarding Sword & Board, Dual, and 2-Hand: Dual wielding and Two-hand will always deal the same DPS if all the weapons are of the same tier. A level 70 mainhand + level 70 offhand will be the same as a level 70 two-hand. Both setups do 150% of mainhand damage(mainhand is 100%, offhand is 50%, two-hand is 150%). So in that regard it does not matter. Sword & Board(Sword and Shield) will always do less damage if equivalent tier, roughly 33% less damage. Unfortunately EOC has made shields mostly useless as far as I know. They are only useful against Dragons if you don't have 85 Herblore for Super antifires.
Before EOC I used Whip + DFS, now I usually use SGS unless I am fighting Dragons.
I've found shields are mainly useful when I need extra HP or for dragons as I mentioned above.
As for shield abilities, only ever have them on your bar if you're wearing a shield and plan on fighting with it. And then you probably only want basics to build adrenaline.
As for Momentum;
Whenever I do slayer I use it because it's just easier and, quite honestly, just as effective and just as fast as abilities. I have fought side-by-side with ability users and we kill things just as fast - using the same quality gear(sometimes same exact gear). In PVP I have fought ability users while using Momentum and won. In one case I was using the SGS and Momentum against an ability user with drygores, he won by ~100 HP.
If you do use Momentum, I would suggest a Two-handed weapon. Personally I don't like one-handed weapons anymore(stark contrast from my pre-EOC ways) because they just don't hit hard enough. Two-Hander + Momentum works well and can really take people by surprise. Not sure if you know this, but Momentum occasionally auto-uses some abilities. When it does this it does so immediately before a normal attack, essentially giving you two attacks in one game tick.
I'll say this:
While I prefer Momentum and Shane of course prefers the Ability system, both are equally effective on average
. There are places where they each excel and where they each fail.
A Simple Rule wrote:
If the creature is a boss or likely to last more than a few hits(a demi-boss creature) then use the ability system.
If it is a normal creature or you have to kill many small creatures(such as Slayer and some special areas) then use Momentum.
Even though I don't often use abilities, I would suggest getting the ones from the WE merely because they are likely exclusive and you may want them later.