Yes, I have tried both. In fact the battery is very good in the X-T1 and you can double up with a 2nd battery in the vertical grip.
Independent tests such as the recent side-by-side comparison in Amateur Photographer magazine have revealed that their is very little difference in noticeable image quality between the X-T1 and the Df up to about 800 ISO.
I opted for the X-T1 as I wanted to reduce weight and yet carry more lenses with me. I carry a Samyang 8mm fisheye, Fuji 18mm, 23mm, 35mm, 18-55mm, 55-200mm. I'm just waiting for the Carl Zeiss Touit 50mm macro which is fabulous quality for portraits to complete my lens line up. All this plus filters, cleaning gear etc, which is very light and portable in a Lowpro Pro Runner 300 AW (All Weather) bag. Try carrying that lot around with FX lenses all day !
Optically the Fujinon lenses are all very fast and sharp. Most of them have an aperture ring.
The aperture ring is very important to me. Since I started using Nikon SLRs in the 80s, I used the aperture ring on the lens and of course the top plate controls. This for me was another major factor in deciding what route to go down.
My advice would be to anyone investing large sums of hard earned money to try both cameras out before purchasing them and get hands on. Take an SD card with you. Take a couple of images inside / outside the shop using a lens you want both at maximum and minimum aperture. Compare them on your computer at home and think about it.
Having said that, after handling the Df I knew it wasn't for me. I was already blown away by the quality of the XPro 1 and pre-ordered the X-T1 after fully researching it and speaking with Fuji HQ.
The Df is a good camera and I'm familiar with Nikon having used one for the best part of 35 years. However, if I wanted a FF I would have chosen a D4.
The two main things to consider are:
1) Full Frame or portability ?
2) Functionality of familiarity ?
Perhaps the only other thing to consider is that there isn't a perfect camera or solution out there. (For me the X-T1 comes close ). There is always another camera in development or something else on the horizon. You can procrastinate for ages, sometimes it's just better to go and make a decision and start shooting and producing images. The final decision ultimately comes down to your head or heart, preference or bias, or just what's going to work for you in the long run.
Nobody ever said buying a camera was easy!