Overall, the type of Lure Module you buy depends on what Pokémon you'd like to see/catch. If you want to see the the Pokémon that would normally be in your area, a normal Lure Module works well, however;
Glacial Lures increase the spawn chances of Water and Ice Pokémon
Magnetic Lures increase the spawn chances of Rock, Steel, and Electric Pokémon
Mossy Lures increase the spawn chances of Bug, Grass, and Poison Type Pokémon.
- All of these last for 30 minutes
Along with the increase chance of certain types of Pokémon appearing, specific Pokémon have increased spawn rates, as well:
- Alolan Diglett
- Alolan Geodude
In addition, certain Pokémon evolve under with the Lures:
Glacial Lure: Eevee --> Glaceon (Or nickname Eevee Rea)
Magnetic Lure: Nosepass --> Probopass & Magneton --> Magnezone
Mossy Lure: Eevee --> Leafeon (Or nickname Eevee Linnea)
All in all, all of the Module serve there own purpose, and do it quite well. If you need any Pokémon or Types listed above, then you might want to use their respective Lure Module. However, if you just want to get some more Pokémon in general, a regular Lure Module is perfectly fine.
I Hope I Helped!