I'm pretty new at this compared to a lot of folks who would answer this question, but perhaps that gives me mystical noob insight :-o . In either case I own both and find the experience very different between the two.
Which do you use the most?
Probably the Kissbox, since it has the USB plugin. No batteries, love the manual switch, I don't have to worry about swapping out while home, just can smoke while working, etc. However, I keep the 901 and 901 mini in my "travel kit" for work and to use for experimenting with flavors (cheaper atomizers if I gum 'em up... yeah I know Bickford's isn't supposed to but I'm paranoid that way lol).
Which gives best vapour?
Here's where the big difference is... the Kissbox gives tons more vapor, lots and lots of it, but of a completely different quality. The vapor from the Kissbox is "lighter", it feels lighter on the tongue and throat. It also doesn't seem to retain the kick or flavor of the 901, possibly because it atomizes to a higher degree (which would explain why the vapor feels so much lighter). That's another reason I use my 901 for monkeying with flavors, to retain more of it with the heavier vape and get the throat hit I sometimes crave. Which vapor do I enjoy more? Hard to say... more, smoother vapor or less, rougher vapor... they both have their place.
Which is most durable?
So far the Kissbox has been finicky. It's higher quality in design, but seems to need more attention. Sticky manual switches, fussy automatic switches, and such... I haven't had an atomizer go out on my 901 or mini, it seems built like a tank, and comes across as a lot more resistant to my inattention regarding maintenance. I'd have to go with the 901... it's the old beater car you drive hard and throw an oil change into once in a while while the Kissbox is the Porsche that needs regular tuning to maintain that performance.
Which is best value for money?
It depends on the experience. Just getting started, go with the 901 in my opinion. You get almost the same experience for half or less of the price and can work into esmoking. The Kissbox is a better value if you're looking for that top-shelf experience.
If the 901 had a USB version (if it does, omg tellmetellmetellme) I'd say definately the 901 in bang-for-the-buck. However the ability to plug in and not worry about spare batteries while one charges is a huuuuge plus in my book.