Yay my first ever ZB post 
I have both zb3.1 and 3d coat. Total breeze to save out of zbrush and into 3dcoat as .obj, couldn’t be simpler.
From what I can see of topogun, it has has a lot less features than 3dcoat, eg voxol sculpting etc. BUT… Topo is lower cost than 3dcoat, but as both apps have free trial, its worth looking at them both in some depth. 3dcoat can be installed on as many comps as you like, but the licence can be active on only 1 machine at a time (per licence). Simple to activate/deactive online, takes about 30 secs.
I actually had 3dcoat before zbrush, and so far I much prefer the retopo of 3dcoat, as much as anything because its simpler to see the new poly edges as they are drawn, and no faffing about with zspheres.
A good summary might be
For a feature richness list top to bottom
Zbrush
3dcoat
topogun
And no surprise, cost is in same order. All have their place just a matter of what feature/s each user is looking for.
I have beta tested for a couple of programs in the past, and have never seen such a rate of updates and new features than those that are added to 3dcoat. In fact its recently got to the point of me skipping release versions through sheer tedium of installing almost 2 new versions a week at times. The developer seems to be some sort of programming machine the speed at which bugs are fixed. and new features brought out, it’s quite a revelation to see it unfold at such a bewildering speed, I’ve never seen anything like it!
As a slight aside, I use a recently purchased wacom cintique and am right handed. When voxol sculpting in 3dcoat it is a welcome relief that the pesky pop-up panel for changing brush size etc, doesn’t appear right under my drawing hand, as it does in ZB, drives me nuts that 1 
Voxols are a load of fun too, though currently a little wip ish. To date using the wacom I can spend hours sculpting in either zbrush/3dcoat just for the sheer joy of it, I highly recommend the cintique it just feels so “right” in either app.
I would assume there will continue to be a need for beta testing as no doubt topogun will evolve more, if it has any hope of staying viable, in what seems to me to be a recently very HOT area of 3D development thats blistering forward by several companies, no doubt spurred by competition, and an increasing demand in this area.
My 2c worth… try em all extensively before buying.
Ok I am outta here lol. I only came on to see if zspheres2 and goz for pc released yet, man am I stoked about upcomming zbrush features, awesome, more than I could ever dreamed of even only a couple of years ago
Seems not THAT long ago I was using deluxe paint on an amiga and thinking it was fantastic 
Cheers
IK Handel