Hi Siy,
ROM is more important than AD. ROM is the day both of you officially and legally become husband and wife. AD is only the banquet and tea ceremony. In the old days, there is no such thing as ROM. Everything is done on the AD day, hence AD is ver important in olden days. In modern day context, if you split your ROM and AD to 2 different days, then ROM will be more important.
For your question on Mang Nian, I've actually written a short explanation of it some time back in one of the threads here. Here is a "repost" of that thread:
It is certainly not true at all that "Mang Nian" is not good for marriage.
First, we must know what a "Mang Nian" means and why is it call a "Mang Nian". Most of the time, we just know "Oh..next yr is Mang Nian, cannot get married coz not good for marriage and no good to have kids". In acutal fact, we must find out WHY. The WHY is important in everything we do. Whether it be Feng Shui, Bazi or Date Selections, sometimes we must ask ourselves WHY?
Coming back to "Mang Nian", basically a yr is considered a "Mang Nian" coz the 1st day of the Lunar New Year falls after 4th or 5th of Feb. This is the case for year 2010. The 4th/5th of Feb every year is call the "Li Chun", which means the "Beginning of Spring". It is a belief that if the "Li Chun" is before the 1st day of the Lunar New Year, then when the Lunar New Year comes, there is "No Spring" or "No Li Chun" in that Lunar Year. This is why it is call a "Mang Nian".
There is no logic as to why you cannot get married in "Mang Nian". If every person in this world who gets married in "Mang Nian" gets divoced, then yes I am convinced that this is a bad year for marriage. But this is not so. And there are so many couples who get married in "Mang Nian" are living happily ever after, right?
Whether a person's marriage is good or bad is not determined by all these tradition, culture, beliefs or myths. It is determined by your Bazi Destiny Chart, which is the date and time you are born. And along with the Feng Shui of your house which will affect and influence your life as well as our own decisions and human actions that we make daily, these will "form" our Destiny. These 3 factors are also what is commonly known in Chinese as "Tian, Di, Ren", which means "Heaven, Earth, Man", the cosmic trinity that governs our life and destiny.
Coming back to "Li Chun" again, in actual fact, there is still a "Li Chun" every year. It is just a matter of before or after 1st day of the Lunar New Year.
And "Li Chun" in Chinese Metaphysics is a very important transition point between each year. It is the date where the Feng Shui of a house changes yearly. It is the date where the Destiny of a person changes yearly. It is also the ACTUAL transition point between 2 animal signs, which means whatever animal sign you are is determined not by the lunar new year but by 4th/5th Feb each year. So this means that if your child is born on the 29th Jan 2009 this year, it is a Rat and not an Ox. Some people make this big mistake and tell their child they are born in the year of the Ox. Imagine the horror 30 years later when he/she grows up and go for a Bazi Destiny Analysis or Wedding Dates calculation, only to realise he is a Rat and not a Ox!
I hope this clarifies your doubts