For my money I wouldn't go past Maltron. Converting from QWERTY to the Malt layout was pretty easy, especially considering that I was working in two different jobs, only one of which let me use the Maltron. In 1990 I trained as a court reporter and spent my time since then (semi-retiring in 2008-9) using the Maltron to transcribe audio recordings at the speed of speech (ie 150-180 wpm) for hours on end. I started initially using a QWERTY, but then I was able to use the Maltron, and the difference was quite marked. I'm now 63 years old, I've been using a keyboard (manual typewriter then computer) since 1967, and although I suffered a sporting injury to my right wrist such that I had to change my mouse hand, I've never had any problems with symptoms of keyboard related injury using the Maltron. The Malt key layout reduces considerably the amount of travel/reaching that my hands/fingers do compared to the QWERTY distribution.
This video shows the Maltron keyboard with the QWERTY layout.
This video shows the same physical keyboard but using the Malt layout
Also notice how much time the QWERTY typist spends with her hands hovering over the keyboard, using major muscle groups in the arms and shoulders, whereas the Malt layout means that the hands can rest on the built in platforms.
The keys are not staggered, so there are no microshifts to left and right to strike keys.
The keytops themselves are individually angled for the optimum finger flex.
The Maltron can be ordered with a built in track-ball set between the two thumb keypads, which reduces the amount of arm movement to operate the mouse, and gives a smaller footprint.
The central numeric/navigation/function keypad can obviously be operated by either hand.
The only con I can think of relates to only one of my keyboards (second hand) and that is the Windows function key impedes my little finger in each hand, but since I never use it, I simply removed the key cap. But Maltron is able to offer a fairly comprehensive degree of customisation of key distributions when you order.
This keyboard is not cheap initially, but since my first keyboard I bought in 1986 is still useable (depending on the plug in the computer - no USB) it's been going without problem for 27 years. It works out at roughly $20 per year.
For a more detailed explanation of my views, my blog might be of use to you.
http://mostergonomickeyboard.blogspot.com.au/Further, this link is to a series of academic papers presented by both Lilian Malt and Stephen Hobday detailing some of their research and findings.
http://www.maltron.com/keyboard-info/academic-papers