The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Tamron 35-100mm F2.8 and the Sigma 90mm F2.8 DG DN C 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. Tamron 35-100mm F2.8 is the longer of the two lenses at 119mm. The Sigma 90mm F2.8 DG DN C with a length of 60mm, is 59mm shorter. Besides being longer, the Tamron 35-100mm F2.8 also has a larger diameter of 81mm compared to the Sigma 90mm F2.8 DG DN C's 64mm diameter.

Tamron 35-100mm F2.8 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, 47% (270g) lighter than the Tamron 35-100mm F2.8's weight of 565g.
Filter Threads
The Tamron 35-100mm F2.8 has a filter size of 67mm whereas the Sigma 90mm F2.8 DG DN C has a 55mm diameter. Larger filters are generally more expensive than the smaller ones given all the other features are equal.
Below are links to the 67mm filters that we recommend you to consider for the Tamron 35-100mm F2.8:
And these are the 55mm filters that we recommend for the Sigma 90mm F2.8 DG DN C:
Lens Mounts
Both the Tamron 35-100mm F2.8 and the Sigma 90mm F2.8 DG DN C 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.
Tamron 35-100mm F2.8 is also available in the Nikon Z mount.
Sigma 90mm F2.8 DG DN C is also available in Leica L mount.
Focal Range
Tamron 35-100mm F2.8 has a focal range of 35-100mm and
2.9X zoom ratio . When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
52.5 - 150mm.
On the other hand, the Sigma 90mm F2.8 DG DN C is a prime lens with fixed focal lenght of 90mm.