Depending on price a used SRM could be a better value. While the accuracy of all three systems is good when they are working, the other two seem to have a lot more issues from what I've seen. Many of the guys I know with Quarq have had issues at some point, both early and later units. My experience with used SRM's have been the equivalent of buying a used tire iron. Not much goes wrong.
With Quarq being bought by SRAM, I'd be a tad leery about them continuing to provide the same level of service, and I've been told they are replacing non-SRAM failed units with SRAM cranks. Worth some due diligence.