I don't think there would be much difference. Using sp's requires no client side support, with DMO you do have to have the supporting libraries on the client. You're not going to see much of a difference in execution speed anyway - you'd have to add a LOT of logins to even care. Optimize what counts, don't sweat the rest.
The advantage of using the sp is simplicity. Pass it a couple parameters, check for bad values, embedded quotes, etc. Using DMO is certainly viable for this, but I think you have to look further before deciding. If all you're doing is adding logins, I think using the sp makes sense. If you're definitely going to use DMO for other operations, then I'd say go with DMO consistently - maintenance programmers will appreciate it.