Better to have more memory or faster memory for SQL server?

Pretty sure the answer to this is going to be obvious, but I'm doing some work on my SQL server (running Windows Server 2008 R2 Enterprise + MS SQL 2012) and have two memory options:

64GB @ 1600MHz
128GB @ 1067MHz

This is a Gen-II Dell PowerEdge R515 with dual hexa-core AMD Opteron 4332 HE's and DDR3-1600. It can support up to 256GB of RAM however due to hardware limitations the more you add the slower it goes. (I'm paraphrasing, and if I were to shell out big $$$ for 32GB modules I could eek out a bit more speed, but that's the gist of it)

FWIW hard drives are 8x 450GB 15,000RPM drives in RAID 10 and NIC is teamed (2Gbps).

This box runs several large SQL databases that are growing by the day (average size currently is around 15GB per db) and is accessed by around 50 users. I know if I was building a game machine I'd want "less but faster",...