I've used red, black & prune plums to make wine & melomel, black plums are my favorite. Sometimes it's a challenge to get good flavored plums from the grocery stores though. Red plums usually have a slightly higher acid content than black plums, and I've used a bled of red & black, but I honestly didn't notice much difference in the final product.
I've only used the prune plums in a blend with black & though it was very tasty, I only made it once, so don't have a direct comparison. I usually "sweat" my plums, meaning that I leave them in a cool, dark & dry place for a few days to a couple of weeks, depending on the condition of the fruit, so that it ripens/softens/sweetens prior to fermentation. I also freeze/thaw the plums for 2 reasons: 1) it makes it MUCH easier to extract the pits, I just squish them with my hands after thawing, but watch out for the pointy end of the pits. and 2) freezing help to break down the cell walls in the fruit, resulting in more juice extraction, hence more flavor.
Current plum prices around here are about $2.79/lb. If the fruit is good & you can process or at least wash & freeze it for later, I'd jump on that $1.00/lb price, that's a smokin' deal!
Regards, GF.