Both are great weapons. I have 2 different slaggas and 2 different Maggies (one of them is Two-Fer)
each is adapted to it's own style. I use the Slagga on my mechromancer to put out fast bullets and lots of them. Since accuracy isn't an issue, the Slagga comes in handy due to it's great damage output.
The Maggie is more adapted to an accurate style. I use it on Zero, and it's very helpful to have on him since he's more geared towards accurate weapons anyway. With or without the Bee, I can one or two-shot many many enemies, pretty much only bosses take time to kill. The wonderful pellet spread of the maggie is great at putting out good damage on a very small location.
In the end it comes down to how you play the game really.
Tried for about an hour to get a Maggie to drop with no luck. Trying to find something legendary that beats out these:
Restructuring Development Hyperion Shotgun 5463*17 38.0 2.5 3.7 12
Extra Large Shotgun Surpreme! Tediore Shotgun 6483*17 30.8 1.0 3.0 11
So far the only shotgun that I found that comes close is the Triquetra. I thought I would go farm for Sledge's Shotgun, but if they're in the damage range of the one shown on the wiki, it's falls short.