There are huge differences between them. Where they are similar is the the projectile kill capability both 155mm that is were the similarity ends. Several of the differences have been detailed above so I wont repeat them but I will comment on the ones that were not mentioned and are from my perspective the key differences.
Although you correctly state that the projectile and weapon effects are the same, you go about comparing a self propelled howitzer with a towed howitzer. The M109 was designed for a high intensity mechanized battle whereas the M777 was designed with a focus on light weight mobility. Two different weapons systems and it is difficult to compare them unless you are referring to the operating environment they will be used in. Each brings a different capability although they both can do the same thing (provision of indirect fire).
Manning ; M109 the minimum manning for basic capabilities is 3 soldiers.
Maybe for the US versions of the M109 (A5/A6) but ours can't operate that way (minimum would be a 4 person detachment) for sustained periods of time. The book states 4 and I'll stay with it.
Time into action from moving to firing : M109 is about 30 secs with old version new version can fire in less than 15 sec with higher orientation : M777 is about 1 min with no orientation
Bringing our version of a M109 from "moving to firing" in 30 secs is doubtful unless it is under controlled circumstances. Later versions of the M109 I agree they can be brought into action much quicker (ie the A6 Paladin) but 15 secs remains questionable as well.
Rounds per min: M109 under normal sustained rate 2 rounds per min with modern gun 4 -6 rounds is not unheard of. M777 1 per min insert edit 1 round per min manual mode I have added this correction for the comparative rates of fire with the M109 "The M777A1 is able to deliver up to five rounds per minute under intense firing conditions and is able to provide a sustained rate of fire of two rounds per minute." On he rates of fire I stand corrected
I have never heard of a sustained rate of fire for the M109A4 as more than 1 round a minute. If I can recall correctly they were capable of 2-3 rounds in the first minute or so and then drop to 1 a minute. 4-6 rds for a Paladin sustained is also questionable but I don't have this at hand (your source?).
On the maintenace issue self propelled guns can and are towed into action I would suggest that an M777 would have the same difficulty getting into action as a M109 if it had a prime mover breakdown.
I don't know what you are trying to prove with this statement. I would suggest that if a M777 gun tractor broke down, you could move the gun to another truck and carry on.
The only issue that would make the M777 a better gun is the cheaper cost to buy it.
Lower cost is an advantage but its hardly the only one. Cost, maintenance, mileage, storage (gun parks), manning, and transportability are all factors in the M777 favour.
As for mobility the need for air mobile guns has diminished thus the portability capability is not as big a bonus as was years ago.
Yes, packing an M109 up in the back of a C130 is done on a routine basis now. I guess we don't need the M777. Or maybe if we strip the armour off the M109 it can be transportable by the Chinooks the CDS keeps talking about.
In a nut shell the M777 is a good gun but it is a poor substitute for a mech gun.
See my comments above. Two totally different reasons to have SP and Tow artillery.
In a perfect world the Arty would have a combination of both M109 and a few M777s for the light capability when needed.
And on that point, I can agree with you.