Question - Seeing as you can swap what weapons you use with certain spells is there really a benefit to using one over another, or should I just stick with whatever weapon is the higher Item Level/Damage?
Ex - my 2 Dual Wield weps are 200 IL and my 2H ones are under 150, is there a benefit to using the 2h at any point on any spells?
Every Arsenal Weapon had a Unique Proficiency Bonus. As you use any particular Weapon, you gain XP with that Weapon. Every Level raises the Proficiency Bonus. Upon reaching LVL 10 with the Weapon, you gain an additional bonus for using that weapon. Level 10 is the max level for Weapons.
Certain Skills can only have certain Arsenal Weapons assigned. Every time you use a Skill with a different Arsenal Weapon, this counts as a “Weapon Swap”. Certain Skills, Aspects, and Paragon provides you bonuses for Swapping Weapons such as Mortal Draw Glyph which gives you Additive Damage on Weapon Swap. This can be as simple as using Frenzy (Dual Wield) then Rend (2H Slashing) back and forth.
As you level it will be important to Assign Arsenal Weapons and do not use the recommended which defaults to highest DPS. You typically will only care about 1 or 2 skills for Damage at most. The rest are just conditional boosts.
You want to be leveling expertise for as many weapons as possible on your journey to 50 so that you don’t have to do it later when it’s more of a pain, and more expensive to respec to enable different skills.
Yes you should still be using whatever weapon has the best DPS, but don’t get overly hung up on one weapon type unless that’s the technique slot you absolutely need for endgame (in my case I want 2HS for the Bleed)
As others as have said, you should aim to use a variety of weapons to increase your expertise. In the end, your technique slot is most effective if it ties to a weapon type that you are not actively using (e.g. a lunging strike / HotA setup might use 2-handed blunt for all attacks, but the technique slot would be something else, like 2-handed sword).
Note that item level is not always the most relevant piece of information. Slow weapons with high damage stats (e.g. hammers) will often outperform higher IL one-handed weapons when using attacks that aren’t strongly conditional on attack speed (e.g. charge, leap, to some extent death blow).
Skills do not go off Weapon Damage but Weapon DPS. In your example of Charge, assuming you do not care the Expertise Weapon assigned to it (which is unlikely) you will want the highest DPS item, not the slowest and highest Damage. This is a departure from older titles where Slow Weapons with big numbers were best for some things and Very Fast Weapons were better than others.
For Barbarian, Expertise and Conditionals have a bigger role to play than DPS. You will always want your Primary Weapon to have the highest DPS.
In the skill menu where you can change which weapon is assigned to the skill, you can see the impact on damage as you cycle through the weapons… that’s the information I go on.