It really depends. for a cleric, they are functionally identical.
Here are the damage expressions:
Two hander: 2d6 + str
TWF: 1d6 + str, 1d6
Literally exactly the same when you factor in that your to-hit is going to be the exact same number for both. You only get your Divine strike (I think War domain gets that at 8 and 14 like the Life cleric does) on the first hit so that does favor TWF slightly at 8 and more at 14.
The next thing to look at is the action economy. TWF uses your bonus action every round. Whereas GWF leaves it free. You've got some bonus action spells you might want to use.
Lastly, there is feat selection. A melee cleric is MAD (Multiple Ability Dependent) so you might not want to actually pick up a feat and let both your stats run up to 20, but if you could settle for an 18 str, you might want to pick up the feat related to your fighting style. The GWF feat gives you extra attacks, but the TWF feat lets you wield one handed weapons rather than only light weapons and boosts your AC a bit.
Really, looking at the numbers, the decision is entirely up to you and how you want to play the character, they are quite similar. I give the damage edge to TWF, and the action economy edge to GWF. Ultimately though, at high levels, your cantrips might end up doing more damage than your weapon attacks anyways.
Anyone wielding two light weapons can attack with one weapon and use a bonus action to make an offhand attack.
The Rogue's having Cunning Action as an option does not prevent them from instead choosing other bonus actions. The restriction only applies to Cunning Action itself - if the Rogue chooses to use Cunning Action, only Dash, Disengage, and Hide can be used. Otherwise they may use any other Bonus Action which is available to them. This distinction is there because the Thief and Arcane Trickster archetypes later expand the options provided by Cunning Action.
The bottom line:
- The Rogue needs the Dual Wielder feat to use Two Weapon Fighting with a rapier, since the rapier is not a light weapon.
- The Rogue can use either Cunning Action, or an offhand attack, but not both.
- Having the Cunning Action class feature does not prevent the Rogue from choosing other bonus actions instead.
Best Answer
No, you only have one Bonus Action a round.
Using your cunning action to Dash uses your Bonus Action. Attacking with your off hand weapon also takes a Bonus Action. Only one Bonus action per round so you would have to choose which you wanted to do.
The Feat Dual Wielding allows you to use two non-light weapons, but it still takes a Bonus Action to make the attack.