I have all the Nik plug-ins, onOne's Perfect Photo Suite Premium 7.5 and nearly all the Topaz apps. If it came down to just Nik or onOne, it'd be the former every day of the week and twice on Sunday. I find Color Efex Pro 4, Viveza 2 and Silver Efex Pro 2 to be indispensable and while I don't do HDR as much as I used to thanks to my D800, I think their HDR Efex Pro is better than Photomatix (which I've had for years). onOne has a few things that are helpful and their Resize (based on/in Genuine Fractals) is arguably the best at what it does but overall, I think Nik is far superior.
I wouldn't overlook Topaz Labs software however, as their Clarity and Detail plug-ins are great and I sometimes use Adjust and Simplify for effect too.
I think most, if not all of these offer trial periods after download, so that's probably your best bet.
I hope this helps,
Phil