Signet of Ghostly Might
Communing
1s, 15r
Elite Signet. (5...17...20 seconds.) All spirits you control within earshot attack 33% faster and deal + 5...9...10 damage.
Basically, SoGM is a force multiplier, it multiplies the effectiveness of your other spirits. The damage bonus will increase the average spirits damage output by 30-50%, while the attack rate bonus will increase all spirits damage output and bonus damage from painful bond by exactly 50%.
Signet of Spirits
Channeling Magic
1s, 30r
Elite Signet. Creates three level 1...10...12 Spirits (60 seconds lifespan). These Spirits deal 5...21...25 with attacks.
Unlike signet of ghostly might which depends on having other skills to provide several spirits for you, signet of spirits just lays a down 3 average damage spirits and does not rely on outside skills except painful bond to create powerful damage output. You cannot do much to increase it's power but you do not have to in the first place.
Brainstorming:
- SoGM will take 2-5 spirits and give you the offensive power of about 3-9 spirits. Whereas SoS will take 0-5 spirits and give you the power of 3-8 spirits. SoGM should start to pull ahead once you have 4 offensive spirits on your bar.
- A communing rit just needs communing to be a solid spirit spammer, It provides 4 good general purpose damage spirits and they have access to vampirism. If they team up with a channeling rit, the channeling rit can take painful bond duty and that leaves the communing rit a lot of free att points and a free skill slot. They can't get e-management from communing though.
- A channeling rit really should put a lot of points into either communing or restoration magic to be a solid dedicated spammer. They can either use communing to get more spirits on their bar, or restoration magic (if teaming with a communing rit) to get spiritleech aura which will pretty much double the damage of vampirism and bloodsong.
- Signet of spirits is still good damage as long as you have high channeling even if the rest of your bar isn't dedicated to spirit spamming.
Whatever, let's just get to the builds.
For these builds I will assume that the ritualist is going to be using it for general purpose PvE play. This means they shouldn't rely on outside help for synergy, e-management, etc... and will have to take summon spirits because you can't afford not to be able to move your spirits in a normal PvE group.
I'm also gonna go ahead and give each build 2 superior runes because it's not hard to avoid aggro as a spirit spammer, just get some nice hp gear and learn good positioning.
SoGM
16 Communing, 15 Channeling
Signet of Ghostly Might, Pain, Anguish, Disenchantment, Shadowsong, Painful Bond, Summon Spirits, Spirit Siphon
Spirit damage after SoGM: 42, 31, 31, 31
After the addition of painful bond: 42+20, 62+20, 31+20, 31+20
Total: 246
Account for attack rate after IAS: 246 / 1.34 = 183.6 DPS
Without painful bond: 100.7 DPS (123.9 if anguish is hitting a hexed foe)
vs.
SoS
16 Channeling, 15 Communing
Signet of Spirits, Pain, Bloodsong, Anguish, Disenchantment/Shadowsong, Painful Bond, Summon Spirits, Spirit Siphon
Spirit damage: 26, 26, 26, 30, 26, 20, 20
After painful bond: 26+21, 26+21, 26+21, 30+21, 26+21, 40+21, 20+21
Total: 341
Accont for attack rate: 341 / 2 = 170.5 DPS
Without painful bond: 87 DPS (97 if anguish is hitting a hexed foe)
Actual DPS will be somewhere between the without painful bond DPS and the max DPS because you can't really assume all targets will have painful bond on them all the time. Either way the SoGM rit does pull ahead a little.
But what if you combine them?
SoS
16 Channeling, 15 Restoration
Signet of Spirits, Bloodsong, Vampirism, Spiritleech Aura, Painful Bond, Great Dwarf Weapon, Summon Spirits, Spirit Siphon
Spirit damage: 26, 26, 26, 26, 20
After spiritleech: 26, 26, 26, 46, 40
After painful bond: 26+21, 26+21, 26+21, 46+21, 40+21
Total: 269
Accont for attack rate: 134.5 DPS
Without painful: 82 DPS
Less raw damage than either of the above builds, but can maintain GDW on several allies pretty easily. Could take plenty of other skills instead of GDW, maybe gaze of fury if you make sure to only use it on spirits that have already recharged or enemy spirits.
With...
SoGM
16 Communing, ?
Signet of Ghostly Might, Pain, Anguish, Disenchantment, Shadowsong, Optional, Optional, Summon Spirits, Spirit Siphon
Spirit damage after SoGM: 42, 31, 31, 31
After the addition of painful bond: 42+21, 62+21, 31+21, 31+21
Total: 250
Account for attack rate after IAS: 250 / 1.34 = 186.6 DPS
Without painful: 100.8 DPS (123.9 if anguish hits a hexed foe)
Pretty much the same damage as the solo SoGM build, but has room to play around with the two optional slots and the remaining attribute points. One of the optionals should be e-management so probably do high channeling or high spawning power and take boon of creation or spirit siphon. The other can be a PvE skill or anything in the attributes you chose.
Conclusion:
If you have 1 spammer in your pug, either is great, but the SoGM does technically have more potential.
If you have 2 spammers in your pug, both are great. They can't reach double the spirit damage of 1 rit but they can have room to make up for it with things like great dwarf weapon.
I feel like I just wasted a lot of free time over-thinking this. Oh well, at least I didn't decide to do something like write a comprehensive report on water magic snares, that would take forever.
N

Including Bloodsong isn't the greatest idea, and is where your calculation went wrong, as Bloodsong doesn't actually do damage and doesn't get the SoGM damage increase (the same holds true for Vampirism).
I do know about it, but I usually use bloodsong anyways because it lasts long, is tougher, and it's only a 5 damage difference. I did forget it when I made the post though.