I have used all three of the programs: OnOne, Topaz, and NIK, and for _me_ (YMMV), it's a no-brainer: NIK's are the most useful for me.
1. Topaz I like, yes, (and I think they are well engineered), but I don't like the "Snap" actions for having to save experiments, and I find these plug-in's harder to use (I still don't know exactly which slider does exactly what...and how much). Also, I have had the masking program crash my system numerous times.
2. OnOne Suite: talk about crashing. With the exception of Perfect Resize, virtually every one of their programs has crashed my system at one time or another. Much of that had to do with their activation program. Anyway, I finally gave up on getting a fix for their masking program. I would definitely not want to depend on using this within Photoshop _as an alternative to Photoshop_ now that OnOne supports layers, but, hey, maybe that's just me.
3. NIK: I have owned and used this suite for several years, and it has been rock solid in every program. I use _all_ of the programs in this suite, and they are all well engineered and fast. I also like the stacking and ability to reorder the stacking of filters. I always attend their demos at Photoshop World, and they always have the most professional and competent personnel (Dan Hughes).
What will become of NIK after the Google acquisition? Who knows, but I can't believe that Google paid a billion ("with a "b") dollars just to let this program die on the vine. Although I don't necessarily trust Google, I pretty much do trust their affinity for money, and their repackaging and reissue of the suite as The Nik Collection at least convinces me that were conscientious enough to put in many man-hours to rebrand it as theirs, in order to _make money_. Filthy lucre is a great motivator, it seems
Next week is Photoshop World in Orlando, and each of the three demo booths is about ten feet apart, so I hope to "interrogate" Dan Hughes at the NIK presentations

Anyway, I did get--and install without any problems--the free-to-existing-owners Google version. Works just the same, but runs faster now on my machine (Mac Pro 5,1, 24 GB ram). I will also, as always, go to the Topaz and OnOne presentations to see what is "new" there and will post what I learn. Maybe their suites run fine now, but I don't have the time to experiment with their software while they tweak it "to perfection."
Hope this helps.