Another vote for the 70D here - but I would use the money saved compared to buying the 6D to buy a 600EX-RT (and also a springy extension cord (I'd recommend Vello branded items) and a small softbox (can buy of quite feasibly home-make one)
), as I think it would be a hugely valuable addition to your workflow and creative options. (The reason I suggest the 600EX-RT rather than the 430EX is it's A LOT more powerful, which is quite helpful once you start shooting with a soft-box, and I'd preference a 600EX-RT to a 580EX II as they controls are a lot more user-friendly on the newer model, but a secondhand 580EX II could save a few dollars if you can live with the controls.)
For your macro work, using flash will give you much better DOF options, and prevent the need to crank up the ISO (which is where the crop-sensor will fall down quickly compared to the Full-frame 6D). Also, you'll find the flash will make the shots very crisp, as the duration of the flash burst is very short (around 1/8000th at full power from memory). This is expecially true if you utilise the "invisible black background" technique - where even though the flash-sync exposure in the camera is 1/200th, because the main exposure is setup so it'll only record a blank black exposure from the natural light (eg: ISO100, small aperture, 1/200th), when the flash exposure is added - at only a fraction of that speed (again, 1/8000th at full power) - it greatly reduces the likelyhood of subject motion blur or camera shake. It also allows you to play with the light balance between the subject and the background, and how close or far you move the light (or size of the soft-box) will change how soft or harsh the lighting is. Some examples of mine from way back:
Black Background Technique:
Flash lighting subject and surrounds:
And mixed light (Natural and Flash combined):
The AF in the 70D will be miles ahead of the 6D (I assume it is the same as what was in the 7D, and that was/is excellent). If you start (continue?) playing with extension tubes, you'll probably find manual focusing easier for the real close-up shots though.
The flippy-screen is a bonus, It's one thing I often wish my 5DII/5DIII had. And another perk of staying with the crop-sensor is the DOF for a given aperture effectively is a bit deeper (not technically correct, but in practical terms it as if it is), so you don't have to battle narrow DOF with a full-frame.
FWIW, I wouldn't recommend a Fuji for you if you're still doing a lot of Macro shooting. The one macro lens doesn't focus particularly close, tubes are available - but the ones I bought felt like they were going to tear the lens electrical contacts out of the body, flash integration is very basic, no Fuji-branded flash offers anywhere near the same output as the 600EX-RTs, flash extension cords and remote triggers are basic at best (if available) and the "smeary" nature of the fine (pixel-level) detail from the Fuji sensors does not lend itself to macros. It's not impossible, but nowhere near as convenient as the Canon D-SLRs. They are still an excellent and capable camera, but not for macros IMO.
In regards to buying an UWA lens, I would highly recommend getting something with a filter thread as opposed to the bulbous front element and permanent lens hood of the Sigma 8-16mm, as adding on a CPL or ND (or plate-style grad NDs) can give you a lot of extra flexibilty and creative options down the track. I think the Tokina 11-16mm f/2.8 is probably the pick of the crop-frame UWAs. While it looses out a bit of focal range at the wide end, it seems a markedly better optical performer than the rest of the 10-24ish pack (most of the others are pretty soft/diffracted in the corners of the frame from what I've seen/read).
Hope that's some help!