For me, in order:
1. Sound quality
2. Workflow & features
In my former life up to about 10 years ago, I was the tech support/configuration guy for a pro audio distributor (the Irish PT distributor at the time) and my rig was Logic 5.5 PC with a Digi 002R (and PTLE, which I never used). I had no desire to follow Emagic into Mac land when they decided to sell to Apple and abandon their PC users back in 2002. I stuck with Logic until 2005, and by then the time had come to change to something that actually supported newer plugins.
PTLE was a no-go because I really didn't like the sound and I had already been using Logic as a front end for several years (previously with a Digi 001). Also the 002 hardware was just not good enough for me any more, and I felt that the deliberate omission of delay compensation was an insult to professional users. In my book, that's not the way to persuade me to buy PTHD. So, instead Digi got none of my money! I sold the 002 and got a Lynx AES16/Aurora 16 combo.
I tried Nuendo and didn't like the interface at all. Inserting a plugin should be possible with one click, not by opening a panel, turning on a power button, then opening another panel to finally get at a list of plugins. Maybe it's changed since, but that's how I remember it at the time.
A producer friend uses Sonar, so I'd seen it in action and I'd had a chance to spend some time with it. 'Nuff said...
That left Samp and SAW. I very much liked the sound of both, but the SAW interface was too weird and cartoon-ish for me. There was also something odd to my ears with the stereo imaging in certain circumstances, where Samp's imaging was always perfect to my ears.
And so the decision was made, and I became the proud owner of Samp 8 Pro!
For the first few months, I had to 'transition' into using Samp regularly, but the great thing for me was the OpenTL export from Logic and the free EDL Translate, which allowed me to migrate my Logic projects into Samp without too much fuss. No question about it, Samp absolutely blew Logic 5.5 away in the sonic department, and I couldn't believe how much better my Logic recordings sounded playing back in Samp.
FWIW, those first few months involved a lot of head scratching and hair pulling, and I felt like an idiot most of the time as I started to learn the ways of Samp. But, like anything else, if you commit to something, you'll get there. And then one day you realise that you're now comfortable and pretty fast, and you don't look back - except to gloat.
Hope this helps!