Since the Data++ version is claimed to fit in a 90MB /sytem partition, I assume that in this version a large part of the ROM has been moved to the sd-ext. This allows you to use a smaller /system and hence have a larger /data partition (hence the name). I'm not sure (because I've only read the first couple of posts, nothing more) whether the Data++ also includes a2sd, or just assumes you will keep your apps in the enlarged data partition. It's probably in the LeeDroid thread somewhere.
If you are not S-Off then it's easy - you want the a2sd version, no argument.
If you are, and are using a custom mtd or an alpharev table with a small /system, large /data, you could try Data++. Which is better will depend on which works best for you (in terms of speed, number of apps you need to install). I've never used either, so cannot offer any experience there.
