Armymatters said:..... i.e. the ability to keep up with armoured units, and to fire on the go. ....
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.
Manning ; M109 the minimum manning for basic capabilities is 3 soldiers.
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
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
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.
The only issue that would make the M777 a better gun is the cheaper cost to buy it.
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.
In a nut shell the M777 is a good gun but it is a poor substitute for a mech gun.
In a perfect world the Arty would have a combination of both M109 and a few M777s for the light capability when needed.
???Sorry I deal in reality