I’m going to second Joseph.

I use both pretty routinely in a lot of bulk portrait work. There is nothing in OnOne that matches the control points of Nik. There is nothing in Nik that lines up with many of the OnOne specialized tools (like Batch, Portrait, Resize, etc). So for me, both are well worth the now modest investment. The overlap is in their effects/presets solutions for B&W and color. If you do a lot of the same kind of thing again and again, both suites are absolutely invaluable. I was sold on Viveza the first time I saw control points and Perfect Portrait saves me oodles of time on every job. Both are also frustratingly buggy when you least expect it. I regularly shake my fist at Google and OnOne. 

If you occasionally tinker, I’d say save some money. Take a closer look at learning to get the most out of the Aperture controls (especially brushing all the adjustments) and get the Macphun software when they have a bundle or sale on. Macphun has really created some cheap, amazing, and accessible tools. You won’t find as tight of integration with each of their tools but they are powerful while keeping it sort of fun.