I like the Direwolf20 because:
1: The mods in it are closer to my ideal set of mods then ultimate
2: Slowpoke messes with some things in the ultimate pack, like where and how often ores spawn, and disables some extra biomes where direwolf20 has default configs for the most part
3: Direwolf20 is somewhat of a mod expert, he has been playing with lots of tech/magic mods for a long time. he has contact with most if not all of the modders and knows a lot about the mods themselves, so I definitely trust his choice in mods. Its also more similar to forgecraft where most of the FTB modders play.
4: It doesn't have gregtech, I personally don't like gregtech, maybe you will, its definitely worth taking a look at.
5: Since when I started playing the direwolf20 pack, chargepads twilight forest, extra bees, modular powersuits, thaumic bees, reliquary, and applied energistics were added. Which shows that direwolf20 is always looking for new cool mods to spotlight and even add to the pack. I feel like he is slightly more likely to add new mods to his pack then slowpoke