Which one do you think is the fastest toward union with God
Neither. If you obsess over the destination, then you miss the point of the journey. If you chase something external, then you will never find it.
It's not because they are just in similar locations but because they have similar traits. For example, the Lower Dan Ti'en is the place of great power in Qi Gong theory. It is where we settle the mind and gather and emit power from, similar to:
"...The pure knowledge given by this (Svadisthana) chakra is not mental, but it is direct perception of the Reality, that can be felt in our palms and indicates our subtle blockages. Also this is the center of pure, steady attention and power of concentration." (http://www.adishakti.org/subtle_system/swadhisthan_chakra.htm#sthash.0DEmbx1K.dpuf)
The Chakras are also in correspondence to certain glands, and other things. Correspondence does not mean they are the same, it simply means they have similar qualities and purposes.
Besides that, they also have different jobs. You cannot cultivate Qi into the Svadisthana the way you can the Lower Dantian. If you try, you're going to cause a lot of problems. They are also in different physical locations. Svadisthana is located in the spine, as all of the chakras are located in and along the spine. The Lower Dantian actually encompasses the entire lower abdomen region, with a "core" point that is located at the physical center of gravity. And that is the important bit. The dantians are not in the same place for everyone; their location depends on body structure and individual anatomy, and this is most especially true for the Lower Dantian. It can be lower or higher, more forward or more backward, and all kinds of things, depending on your sex and height and even your weight. The chakras, in comparison, are generally in the same places along the spine - specific nerve clusters that activate by stimulation from Shakti.
One main thing that does connect them is Chong Mai though, the Thrusting Vessel, which extends from Huiyin (where Shakti lives and Muladhara awakens) to Baihui (where Sahaswara blossoms) and runs along all seven chakras and all three dantians. But that says that Chong Mai is bigger than most people think and is more important than most people think, not that the dantians and the chakras are the same.
I did get the Dan Ti'en wrong of where Light Air Kung Fu stems from, not the Middle, but the Higher. Makes sense, though, considering it is sort of a Shen Gong exercise. Concentrate the Qi low the Qi grounds, concentrate the Qi high the Qi rises... It is mentioned (very, very) briefly in this video:
Wushu (Baguazhang): Jerry Alan Johnson - The Eight Animal School of Pa Kua Chang video on YouTube at about 18:38.
I don't think I have ever seen Qing Gong referred to as a Shen Gong exercise before this. I do not practice it that way, and none of the people I know to also train Lightness practice it that way.
I exercise Qing Gong as an application of Liu He. Yi, Xin, Qi, and Li move together as one unit. Because of this, I can circulate the Qi in my Dantian to strengthen my body in certain places, and focus my Yi and Qi to help me control my body in certain ways. This is why the lower dantian is most important to my practice - not just because it is the Qi center, but also because it is the physical center of gravity.
Specifically, I was taught that "all movement should come from the Dantian." Meaning that when I move (especially when consciously practicing), the movement is initiated in my core and spreads out from there. Doing this, I have a great deal of control over balance and lightness. Even in mid air, if I am falling wrong, I can move from my dantian and control my fall any way I like. This has given me a great deal of freedom to do all kinds of movements, such that even most Parkour (which is an excellent way to exercise Lightness) is almost too easy. Especially when combined with the Flow principle I learned from practicing Parkour, and a few other things.
Of course, having done a lot of odd things with Shen Gong, I have no doubt that Lightness can be achieved that way. The Lightness of Bardon's Air Accumulation can probably be considered that kind of practice too, if you look at it a certain way. But it's probably a lot easier to learn physical movement arts by learning correct physical movements, and maybe using Shen Gong to enhance that. Qing Gong as a Shen Gong practice alone would take a lot of time and work to achieve.