The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Samyang 135mm F2.0 ED UMC 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. Samyang 135mm F2.0 ED UMC is the longer of the two lenses at 122mm. The Sony E 55-210mm F4.5-6.3 OSS with a length of 108mm, is 14mm shorter. Besides being longer, the Samyang 135mm F2.0 ED UMC also has a larger diameter of 82mm compared to the Sony E 55-210mm F4.5-6.3 OSS's 64mm diameter.

Samyang 135mm F2.0 ED UMC vs Sony E 55-210mm F4.5-6.3 OSS 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. Sony E 55-210mm F4.5-6.3 OSS weighs 345g, 58% (485g) lighter than the Samyang 135mm F2.0 ED UMC's weight of 830g.
Filter Threads
The Samyang 135mm F2.0 ED UMC has a filter size of 77mm 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 77mm filters that we recommend you to consider for the Samyang 135mm F2.0 ED UMC:
And these are the 49mm filters that we recommend for the Sony E 55-210mm F4.5-6.3 OSS:
Lens Mounts
The Samyang 135mm F2.0 ED UMC has the Sony Alpha lens mount whereas the Sony E 55-210mm F4.5-6.3 OSS has the Sony E lens mount. Some of the latest released cameras that are compatible with these mounts are , and for the Sony Alpha Mount and Sony ZV-E10 II, Sony A6700 and Sony FX30 for the Sony E Mount.
Samyang 135mm F2.0 ED UMC is also available in the Canon EF , Canon EF-M, Four Thirds, Fujifilm X, Micro Four Thirds, Nikon F (FX), Pentax KAF, Samsung NX and Sony E mounts.
Focal Range
Samyang 135mm F2.0 ED UMC is a prime lens with fixed focal lenght of 135mm. When it is mounted on an APS-C sensor camera with 1.5x crop, it provides a 35mm (FF) equivalent of
202.5mm.
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.