Before I begin, bear with me...I'm not very good at reviewing alcohol, so you're not going to find a long, detailed analysis. Instead, I will provide you with the elementary thoughts and opinion of a "regular Joe."
Both whiskeys were sampled neat over a bit of ice. The Black Label was taken from the freezer - the Single Barrel at room temperature. I drank water between each sip to try to cleanse my pallet (admittedly, a burning sensation from the alcohol lingered on my tongue).
I first sampled the Black Label. The first thing I noticed was a pleasant, carmel-like scent that greeted my nostrils when I brought the glass to my mouth. I never noticed this before, but it is quite pleasant. When the liquor hit my tongue, I held it in my mouth for awhile and tried to identify any distinct or interesting flavors. Honestly, I didn't taste much until I started to swish it around my mouth. This seemed to open up the whiskey and I was greeted with a nice mix of flavors that I honestly couldn't identify.

Sorry, but my pallet just isn't sophisticated enough at the present time. One thing I did notice, however, is that the flavors were not incredibly balanced (compared to the Single Barrel which I will get to shortly). While this didn't create an unpleasant experience, it left much to be desired. Taken everything into consideration (versatility, price, taste, availability, etc), I'd rank this a 7.5/10. It remains my favorite liquor, but that's because it's a necessary component of my favorite drink - Jack and Coke. Sampled straight, it's nothing to write home about.
Moving on to the Single Barrel, I was presented with a much more enjoyable experience. Whereas the Black Label seemed to suffer from an imbalance of flavors, here everything "worked." The drink is incredibly smooth given its 94 proof ABV and even a relatively inexperienced drinker wouldn't have too much trouble drinking this straight. Overall, I rate this a solid 8.5/10. I was tempted to rank it higher, but its price kept it under 9. Nevertheless, it's definitely worth checking out if you get the chance.
Bottom Line
If you're just going to mix the whiskey, stick with the Black Label. While there is a noticeable difference when you drink a Single Barrel and Coke, the difference does not justify the price (this is coming from someone that makes their J&Cs at virtually a 50/50 ratio). If you want to drink it straight, the additional cost of the Single Barrel may be worth it. Cheers and happy drinking!
Note: I wanted to compare this with Gentleman Jack (the line that falls between these two), but I I didn't have any on hand and it wouldn't be fair to compare it without sampling it at the same time.