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.
The best way that I can see you improving your combat effectiveness is through the specific spells you choose.
As you had said, money is sort of tight in your campaign, so you need to make due with what you have at your disposal naturally. Luckily, this isn't hard considering that you have a +4 Cha score (Giving you anywhere between 1-4 additional spells per day at your current bonus). So considering your want to improve how well you do in combat, focus on preparing combat buff spells
These are the spells that I think you should prioritize (keep in mind that I only have the core rule book on hand):
+Bless Weapon(1st): A very situational buff, but powerful when used against the right enemies (ya did good on this one)
+Magic Weapon(1st): Prepare this instead of Bless. Bless may give you a bonus against fear, but Magic Weapon increases your damage as well as your attack bonus.
+Bull's Strength(2nd): I know that you put most of your stuff into Ranged Weapons, but everything on the list available to me is either a buff you don't need or something that seams too underhanded for a "rock star" (also, I don't think you need a spell that lets you take half of the damage that your friends take when there is a fully-functioning tank). As you may already know, Bull's Strength increases your Strength Ability by 4, so at the very least, you increase your Combat Maneuver Scores. This means it's easier to absolutely wreak an enemy's day by affecting how they move, inflicting statuses on them, or simply moving through them.
+Greater Magic Weapon(3rd): THIS is one of the key spells you should prepare when you get into the upper echelons. It's like Magic Weapon, but it gives you additional bonus amounts for every four levels your character has. Also, this spell specifically states that you can cast this spell on up to 50 arrows in the same quiver; this means that if you have to switch to another bow, or if your party also invests in bows, the spell is used to its maximum potential between multiple weapons.
+Prayer(3rd): Anyone in a 40ft radius of you receives a +/-1 to every roll depending on their loyalties; That's an increased advantage of 2 against whatever enemies you are fighting!
+Dispel Chaos/Evil(4th): these two spells give you +4 against the attacks of the designated alignment and automatically dispels effects cast by said creatures or spells that are of that alignment
This last one is just in case you attempt to build physically or your Fighter is being cornered by a Demon or something...
-Holy Sword: PROS: weapon automatically becomes +5 AND deals an additional 2d6 damage to Evil Creatures. CONS: Can only be applied to melee weapons and negates the powers of whatever weapon it is used on (that last bit could be fun to use against an enemy using an all-powerful cursed weapon, but again, this is a bit situational)
I hope this helps.
P.S.: Your party should invest in Heavy Repeating Crossbows once you hit level 10
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Best Answer
Feats
Here are a few options to increase your chances to land a spell on enemies:
Spell Focus: Choose a school of magic and apply +1 to the DC of all spells of that school.
Greater Spell Focus: The previously chosen school is now +2 DC.
Spell Perfection (at 15th level): Choose one spell to master and double the bonus from Spell Focus. Won`t be available anytime soon, but should be listed here for reference.
Heighten Spell (Metamagic): This will allow you to cast spells using higher spell slots, which automatically increase their save DC.
Focused Spell (Metamagic): Increase the spell level by one to increase the DC by 2 on a single target of a spell that affects multiple targets.
Debuffing
Yes, you should consider ways to debuff your enemies that allow no save, or spells that lower their saves:
Cause Fear: One creature up to 5 HD becomes frightened if they fail their save (which also applies -2 to checks from shaken), or if they pass, they become shaken for one round.
Prayer: No save, debuff their attack rolls, damage rolls, saves and skill checks while buffing your party at the same time.
Sacred Space: Sancities an area to increase all good spells DC by 2, or against evil outsiders, which also debuffs their attack, damage and saving throws by 1.
Or effects that apply the Shaken condition (-2 to saves):
Intimidation: Intimidating a foe is the easiest way to make an enemy shaken, and can be done even by a fighter or rogue in your group, but preferably, should be someone that invested a few feats on it.
Doom: Will save or shaken.
Or effects that apply the Sickened Condition (also -2 to saves):
Be aware that creatures immune to mind-affecting effects will be immune to fear effects.
Other Classes
There are options from other classes that are available either as multiclass or by other party members that you could benefit from, but are also readily available (at first level):
Evil Eye (witch): Applies -2 to either attack rolls, skills or saves, even if they pass the save, the effect lasts for one round.
Misfortune (witch again): Target that fails their will save will have to roll twice and take the best result on any ability check, attack roll, saving throw or skill check for one round.
Spell Bane (inquisitors): While your bane ability is affecting a creature type, all your spells DC increase by 2 against that creature type.
If we knew what was your deity and cleric domains (or even the composition of your group, afterall it's a team), we could also suggest some domain abilities that could help, but i believe you got it after all these ideas.