They're quite different. So it really depends on what you want. Here are two pics from Google to illustrate:
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Shape: The GST will sag from weight and lose shape over time - although there are tips in the Chanel forum for holding this off for as long as possible (i.e. using shaping inserts). The Dior soft is unstructured to begin with, so there is no "stiff" shape to maintain. Also, the Dior soft has a rounded base, so sagging will not really change the look of the Dior.
Against the body: The GST is like a box against the body. The entire body of the bag is stiff, so a common complaint is that it doesn't lay snug under one's arm & the straps slip off the shoulder. (A common tip is to fold one strap over the other, but this doesn't always work for everyone.) B/c of the boxy shape, the GST is also prone to more everyday wear and tear from bumping into walls/doors, etc. The Dior soft has a slimmer profile than the GST. So it can be tucked under the arm easily, and you can comfortably drape your arm over it.
Also, the GST will "stand" by itself, but the Dior Soft will not.
Opening: The GST has a wide opening that is the same length/width as the base. The Dior Soft's opening is narrower than the length/width of the bag by a smidge.
Exterior appearance: The GST is in caviar vs. patent Lady Dior Soft. They look quite different. The GST is a bit more casual, while the Lady Dior Soft looks a bit "dressier" (although it's still a casual bag).
HTH. GL.