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 Sony E 55-210mm F4.5-6.3 OSS 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. Sony E 55-210mm F4.5-6.3 OSS is the longer of the two lenses at 108mm. The Sigma 90mm F2.8 DG DN C with a length of 60mm, is 48mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 64mm.

Sony E 55-210mm F4.5-6.3 OSS 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 50g (14%) lighter than the Sony E 55-210mm F4.5-6.3 OSS which has a weight of 345g.
Filter Threads
The Sigma 90mm F2.8 DG DN C has a filter size of 55mm whereas the Sony E 55-210mm F4.5-6.3 OSS 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 Sony E 55-210mm F4.5-6.3 OSS:
Lens Mounts
Both the Sigma 90mm F2.8 DG DN C and the Sony E 55-210mm F4.5-6.3 OSS has the same Sony E lens mount. Some of the latest released cameras that are compatible with the Sony E lenses are Sony FX5, Sony A7R VI and Sony A7 V.
Sigma 90mm F2.8 DG DN C is also available in the Leica L mount.
Focal Range
Sigma 90mm F2.8 DG DN C is a prime lens with fixed focal lenght of 90mm.
On the other hand, the Sony E 55-210mm F4.5-6.3 OSS has a focal range of 55-210mm and 3.8X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 82.5-315mm when used on a APS-C / DX format camera.