They'll recommend ArcEmu, because it has Lua and is "easier to customize". I'm not sure what that's supposed to mean since I don't use it.
I'd recommend TrinityCore. It's a derivative of MaNGOS, but generally is developed faster than MaNGOS.
The main difference between the two would be:
- ArcEmu is very crash-happy, generally due to the database rather than the core.
- ArcEmu is "easier to customize". I don't use it, but that's what is always touted.
- ArcEmu has a Lua engine to load scripts without recompiling.
- TrinityCore has more class spells & talents working correctly.
- TrinityCore, in my experience, rarely crashes.
MaNGOS and TrinityCore basically have different feature-sets, but i'd recommend TrinityCore because it will usually include MaNGOS patches in addition to their own fixes.
In summary:
If you're making a Blizzlike server go with TrinityCore.
If you're willing to put in a bit more work and effort than normal into your [fun]server to have everything working, go with TrinityCore, otherwise ArcEmu.
If you're going to wipe the database and start from scratch, either emulator is appropriate, but you will still have more class spells working on TrinityCore.
If you're just running a server to mess around with a few friends, ArcEmu is easier to run.
If you're capable of patching the core to fix spells, you can go with either emulator without too much of a difference.