You're forgetting about heat. With lower voltage you have to push more in the beginning to keep up. As a result batteries and motor heat up very quickly. This increases the internal resistance and as a result the voltage drops even further!
In my opinion it is allways best to go with higher voltage as long as the difference in runtime does not exceed say 20-30s at 30Amps. The higher voltage, even with the shorter discharge time (runtime is actually the wrong word!), will actually lead to longer runtimes (no need to push from the start!) and will give you a more constant (and higher) voltage throughout the run. The latter is only true if internal resistance is low to begin with, so low internal resistance is, in my opinion, also more important than discharge time.