I've used both in field conditions. I know nothing about SNR or anything else--I'm an idiot. So here is my very amateurish and subjective review of both.
I like the Tinybox because it is ... well ... tiny. The darn thing is the size of a pack of cigarettes.
I disliked the Tinybox because of the jacks and the switches. I really don't need much gain if any, and the jacks were goofy. That said, I understand that it is a custom product, and probably could be made to order. The rechargeable battery deal is fine for most shows but for festivals might it not work for me. I had some clipping issues with the box, but I honestly think it was operator error (me) because of the gain switch (I may have had the setting wrong), as opposed to something inherent with the box. That switch setting was confusing. I have a friend that needs a small economic super stealthy preamp, and the Tinybox would be my first choice for him.
I like the NBox because of the sound and simplicity. When I got back into taping, I downloaded 100 of my favorite shows and looked at the equipment used. About 30% of the equipment was Schoeps > NBox and another 30% was Neumann KM 18x > preamp. The Schoeps > Box rig sounds great. It is also stupidly simple--it has one off-on switch. There is no way to goof up a recording with that box.
I dislike the NBox because it is a battery hog and the Switchcraft plug is very fragile--I've broken a few. It is also bigger. I understand the new boards will have quadruple the run time.
That said, based on my use of both, I picked an NBox. I actually have three of them, one I use, one is a back up and redundant rig, and the third one will be a Platinum.
I think both products are great, I just think one is best suited for a different kind of taping--The Tinybox for super stealthy economic jobs, and the NBox where quality is more important than size. Each to his own, actually. I'd love to get both if they used the same jack. That's my opinion, and the more choices we tapers have the better.