A ring group takes a pilot number and rings the users' base sets. So when their phone rings it may be from the ring group, or it may be a direct call - it's hard to tell (Unless they're close together if the phones ring at the same time it's likely a ring group call). With groups you can also have people go on make busy when they are not available or working a project and not receive calls from the pilot number.
With the multicalls users have a button on their phone and will be able to distinguish the source of the call. They can also put this call on hold and use or answer their base extension.
If there is no particular need either way, with IP sets that have enough keys I tend to prefer the multicall method but there's nothing wrong with having a ring group.