Ask HN: How do you compare CPM and magazine ad rates?

I am looking at purchasing an ad in a magazine vs. a CPM button.

CPM button costs $400 (8 CPM - 160x150 pixels, top right) Magazine 1/4 page ad costs $410 (32k circulation)

I expect market variability, but would appreciate any insights on judging the best value when comparing these ad types. Currently, running an ad in both isn't an option although doing so and tracking the results would be ideal. I am hopeful that perhaps someone has already done this and noticed a significant result.