South Korea fields the M-109 as the K-55 and there are differences.
Some things are trivial such as the K-9 having automatic turret lock as opposed to the K-55's manual turret lock.
There is also differences in the fire control system. I'm not a real arty guy so I can't really explain it but the K-55's fire control system is apparently more hands on as opposed to the K-9. This I have heard from artillery guys as well. The K-9's FCS is more advanced.
Actually, don't just take my word for it. http://www.globalsecurity.org/military/world/rok/k9.htm
Globalsecurity.org claims that it's superior to the M-109A2 in just about every way.
Another source I read a bit for some tid bits: http://koreadefence.net/print_paper.php?number=951
Though I don't think it'll help you much, it's all in Korean.