The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 90mm F2.8 DG DN C and the Sigma 70mm F2.8 DG Macro A for their suitability to various photography scenarios:
Size and Weight Comparison
The size of a lens is a crucial factor to consider when comparing two lenses. Sigma 70mm F2.8 DG Macro A is the longer of the two lenses at 106mm. The Sigma 90mm F2.8 DG DN C with a length of 60mm, is 46mm shorter. Besides being longer, the Sigma 70mm F2.8 DG Macro A also has a larger diameter of 71mm compared to the Sigma 90mm F2.8 DG DN C's 64mm diameter.

Sigma 70mm F2.8 DG Macro A vs Sigma 90mm F2.8 DG DN C Lenses Size Comparison
The weight of a lens is equally significant as its external dimensions, particularly if you intend to handhold your camera and lens combination for extended periods. Sigma 90mm F2.8 DG DN C weighs 295g, which means it is 220g (42%) lighter than the Sigma 70mm F2.8 DG Macro A which has a weight of 515g.
Filter Threads
The Sigma 90mm F2.8 DG DN C has a filter size of 55mm whereas the Sigma 70mm F2.8 DG Macro A has a 49mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 55mm filters that we recommend you to consider for the Sigma 90mm F2.8 DG DN C:
And these are the 49mm filters that we recommend for the Sigma 70mm F2.8 DG Macro A:
Lens Mounts
Both the Sigma 90mm F2.8 DG DN C and the Sigma 70mm F2.8 DG Macro A has the same Leica L lens mount. Some of the latest released cameras that are compatible with the Leica L lenses are Leica SL3-P, Panasonic S1 II and Panasonic S1 IIE.
Sigma 90mm F2.8 DG DN C is also available in the Sony E mount.
Focal Range
Sigma 90mm F2.8 DG DN C is a prime lens with fixed focal lenght of 90mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
135mm.
On the other hand, the Sigma 70mm F2.8 DG Macro A is a prime lens with fixed focal lenght of 70mm.