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 Leica APO-SL 90mm F2 ASPH 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. Leica APO-SL 90mm F2 ASPH is the longer of the two lenses at 102mm. The Sigma 90mm F2.8 DG DN C with a length of 60mm, is 42mm shorter. Besides being longer, the Leica APO-SL 90mm F2 ASPH also has a larger diameter of 73mm compared to the Sigma 90mm F2.8 DG DN C's 64mm diameter.

Leica APO-SL 90mm F2 ASPH 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 405g (57%) lighter than the Leica APO-SL 90mm F2 ASPH which has a weight of 700g.
Below you can find the dimension and weight table of the Telephoto Prime Lenses in Leica L Mount for size comparison purposes.
Telephoto Lenses in Leica L Mount for Size Comparison
Filter Threads
The Sigma 90mm F2.8 DG DN C has a filter size of 55mm whereas the Leica APO-SL 90mm F2 ASPH has a 67mm 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 67mm filters that we recommend for the Leica APO-SL 90mm F2 ASPH:
Lens Mounts
Both the Sigma 90mm F2.8 DG DN C and the Leica APO-SL 90mm F2 ASPH 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
Both lenses have the same focal range of
90mm but while Sigma 90mm F2.8 DG DN C has the fastest aperture of
F2.8, the Leica APO-SL 90mm F2 ASPH's widest aperture value is
F2.