There is very little discussion of lenses on this site because, as our name indicates, we are primarily a Photoshop discussion / education site. I have no personal experience with either of those lenses, or for that matter any lenses other than for full-frame, non-crop sensor bodies.
If you have not already done so, you should search on sites that deal specifically in lens and reviews of other camera hardware. The first two that come to mind are www.dxomark.com and www.dpreview.com. Below are some screen grabs from these two sites. The best of luck.
Also, if you have not already done so, the websites of major, reputable distributors of photography equipment often have a large number of reviews of modestly priced, consumer grade gear like these two lenses.
Although the reviews on such sites are anecdotal, and often by people with very limited experience with a wide variety of lenses of a particular type (i.e., lenses like this are often the first non-kit lens someone will purchase), the large number of reviews available often will give you a sense if there is something seriously wrong with a particular model.
In the case of these two lenses, a quick search of www.bhphotovideo.com showed large numbers of reviews (nearly 400, as I recall), essentially equal pricing, and very good reviews for both lenses.
PS - You should be aware that if you buy a third party lens, while it may work on bodies currently produced by a mfgr (eg, Nikon, Canon, etc.), the mfgr of the body is under absolutely NO obligation to provide backwards compatibility with 3rd party lenses in their future offerings. This has been a particular problem for many Sigma lenses in past years, and is only likely to get worse as more electronic (vs mechanical) features (eg, VR, piezo focusing motors, etc.) are introduced. This is in contrast to lenses made by the same mfgr as the body. These mfgrs will typically do everything in their power to keep the lens-body interface stable, often, for decades, to promote brand loyalty. For example, I can still use lenses I bought in the 1960's on my most modern Nikon DSLR. This is another way of saying that while you may pay less for a 3rd party lens, you may be stuck with a useless paperweight in 5 years, and only be able to sell it at a very low price because few people will want it at that point.