Circle mill would offer the best production in a days work and since ties are the focus you could box out the heart for ties, assuming thats all they will accept, and shuffle the other parts to a bandmill for grade sawing.
The challenge I have seen with cutting ties is moving them. Unless you have a smart set up it can be labor intensive.
There is a sawmill outfit in Farmington, Illinois that makes a circle mill specificly for ties and I thought they promoted up to 100 ties a day from it or more. I will have to check out my lumber mag. and get there add information.