Criteria for choosing samples_per_symbol value

Hi friends,
What is the criteria for choosing the samples_per_symbol value? In
benchmark_tx it is chosen as two. What will the effect be if I increase
its
value such that the the constraints on interpolation value and the usrp
rate
are still met. Apart from reduced data rates what other effect will it
have.
How will the quality of signal change?

Thanks,
Ali