How to choose flip flop type for implementation?
The choice is purely arbitrary. Some flip-flop types can reduce the complexity of your next-state logic. However it is really hard to say which one will be best because you would need to check every flip-flop type with every encoding you could possibly use. 

D flip-flops are easy to use because its excitation is exactly the same as the next state. 

If you plan to use CPLDs or FPGAs to implement your machine use D flip-flops as they will have a D flip-flop built in. their