Thank you for the question. Wildlife management can be a difficult problem and we all want a quick, easy, fool proof solution! Unfortunately, this is just not possible to achieve. Repellents discourage deer feeding by having an offensive taste, odor, or a combination of the two. Repellents generally are not continuously effective, even if listed as such, and a repellent that works in one location may not work in another. Using commercial deer repellents to manage deer browsing in the Landscape are generally more effective when the following conditions exist:
1) Low to moderate deer pressure
2) Light to moderate feeding damage
3) Small acreage
4) Repellents are not being used on adjacent properties
5) Alternative food sources are available.
If any of the above conditions are not typical of your situation, then you should compare the cost of using repellents to fencing systems or other available deer management strategy.
If you decide to go with repellents, remember that repellents are not fences. If deer are hungry, they will eat most anything, including plants treated with a taste repellent. Deer are creatures of habit. Better results may be obtained by changing the repellent from time to time or using a combination of repellents. If you use a systemic product, it is advised that you also use a topical product until the systemic one can be absorbed by the plant, usually for 2-4 weeks. The most important consideration when preparing to apply a commercial deer repellent is the label. Always follow label directions carefully.
The following linked publication of a study at University of Maryland Extension, sort of answers your questions about the two products. The exact products you mention aren't tested, but the ingredients are. In reality, there is no best product. The results show that each product is effective to some degree, but there are some limiting factors. Deer 1, the product that contains capsaisin, also contains other ingredients and was tested for only 1 year. Tree Guard, containing denatonium benzoate, was tested for 3 years so results are more complete. Note that using the product results in less plant feeding than not using it. http://extension.umd.edu//sites/extension.umd.edu/files/_docs/articles/FS810-A_UsingCommDeerReps.pdf
If you have some particularly valuable hosta, you may wish to install a barrier such as fencing to prevent browsing by deer.
Thank you for contacting Extension.