Technically, it's better to have both.
Higher frequency AND lower timings. At the same frequency, lower timings are better. At the same timings, higher frequency is better. At high frequency and low timings, you would be very, VERY happy.
Although what Mr. Maziar said above is true. Having a higher frequency allows your RAM to run with CPUs with high base clocks and tighter BCLK : DRAM ratios. If that didn't make sense to you, you may ignore it.