Id you want to get into the weeds, fireball does an average of 28 damage once versus Spirit Guardians 13/round. In two rounds you are close to equal and in 3 the SG has outpaced the fireball.
I'll be glad to get into the weeds.
So we agree you need to be in Spirit Guardians three rounds to outpace fireball damage.
Since you didn't address any of the points I made about how long you are in Spirit Guardians, I assume you aren't debating them. That (a) combat is over/creatures dead before they get 3 rounds of exposure - considering combats average 3-4 rounds very likely, (b) creatures can leave the area before 3 rounds of exposure, and (c) Concentration can be broke, ending it. Heck, Concentration can be broken by a creature outside the AoE having an action before
any damage is given if the initiative falls out the wrong way since it's only given at the start of a creature's turn. Remember the you need to be close to the foes for them to be in the aura, so you are opening yourself up to attack and the possibility of losing Concentration.
With these in play, it's pretty obvious that getting all of the targets for three rounds is unlikely, and therefore Fireball does more damage on average.
You can also look at damage type....fire vs. radiant and the relative merits of which one is "better" from a monster selection stance.
Sure, I agree with you on this. Casters have a selection of spells, I assume you'll chose a correct one and not use fireball vs. a fire elemental or demon.
You can also have SG not hit friendlies in it's area, even if they move into it after it's cast.
This is a big bonus for Spirit Guardians. Friendlies you designate at time of casting (so you'd need summons or whatever already in play, but that's minor thing.)
On the other side, where you cast it is limited by the caster - if you can't (or don't want to) get to an area you will have limited targets.
Both have times when they can target more foes, this is basically a wash.
Even if enemies run away or avoid the SG are you are still exerting battlefield control in a way instant spells do not.
If we want to look at other aspects besides damage, then requiring Concentration is a huge opportunity cost. It means that you don't have up another Concentration spell so you are accomplishing even more each round than just fireball damage. It means you can't cast it several times overlapping, while if the opportunity presented itself you can cast Fireball three times.
Spirit Guardians is a great spell. It's a staple of the general cleric list. But the idea that a wizard can't match it in damage -- your claim I was answering -- has been shown not to be the case over the course of encounters. Both have times when they will do better if we tailor a situation to them, and that's good for the game.