Since we are in the middle of a 5 day heat wave here in Portland, I didn't really feel like making hot tea as is my usual routine. Instead, I have taken to overnight cold brewing some black teas for the next day.
Yesterday, I drank a dian hong of unclear provenance. Seems to be kind of a standard Yunnan black tea, nothing notable about the variety or anything that I'm aware of. I purchased it a random tea shop when visiting my inlaws just to have something to drink. Generally fine, pretty familiar dian hong flavors w/o anything sour or off about it when I drink it hot/normal (as I've had happen with random buys before). It was made in the style where the roll it up tight like bi luo chun, although still a black tea.
When cold brewed, it is sweet in that sort of spicy dian hong way, although in a mild to moderate intensity. You miss out on some of the floral notes since the nose of the tea is very weak when cold brewed compared to steaming hot, but they are still there on the palate. Very nice as a cold brew, and I think it would work well as the base for a sweet tea if I wanted to go that route. On the lighter side of color for what I'm used to with dian hong.
Today I am drinking some xiao zhong/small leaf hybrid or descendant variety Wuyi black referred to as "gao shan tu cha" or "high mountain clay tea" which I purchased through Yunnan Sourcing. (I know they specialize in Yunnan teas, but this one is a decent one the proprietor must be able to get at Kunming tea markets or something.) As to why it's called that, I don't really know. I suppose it might have to do with the soil being different than is standard for Wuyi (a region famous for its "rock tea" or "yancha"), but I don't really know specifically where it comes from to know if that's the case. Anyway, it's general taste is something like a standard zheng shan xiao zhong or jin jun mei in flavor when I've had it hot. Maybe a bit milder than JJM in intensity, but sweeter.
From the overnight cold brew, it comes out very creamy in mouthfeel, almost milky (not in appearance though). The color is very pale, you could almost think it is a lower grade white tea. Flavor is light chocolate notes, which when considered with the mouthfeel makes it almost seem like you're drinking a light chocolate milk or something. It's definitely got milk/cream sweetness to it as well.
I don't think either is really better per se. I think the dian hong brews out closer to what I would think of as being "iced tea" from before I was into tea as an enthusiast, but in a good way. More "active" or "alive" on the palate, if that makes sense. The Wuyi hong drinks much heavier and richer, which is kind of different in a cold tea. Maybe not what you are looking for as much on a hot day in that sense, but the fact that it's cold is what really matters.