It depends to some extent on what programs you are running - if we are talking productivity software, web browsers and such then probably increasing the amount of ram would be better than faster ram.
Higher speed ram only really matters in situations which are sensitive to memory bandwidth and latency (such as games). That said 16gb is plenty in most situations, going for more is only going to improve performance if you are currently maxing out that memory capacity with what you are doing. You can check this by keeping the performance tab of task manager open whilst working - if you don't have any free ram then adding more should help.