This question is asked and answered several times and if you search, there is a long thread for iOptron. But to shortly answer your good question, it all comes to mater of choice and what you expect or can afford like going for a Porsche coupe or Kia SUV.
I own and work with both so might be able to help you with that.
Few points to remember:
- Both are among the smallest (Best when mobility is the concern and you really can not move long in wilderness with a large tracker).
- Both will do the job perfectly and you simply can't go wrong with either.
- Vixen's built quality is significantly higher.
- iOptron moves twice the weight.
- Vixen has additional modes such as moon, sun and 1/2 speed tracking and you should do the homework to know why you need these features (And they can be really useful).
-iOptron comes with expensive scope, but you should pay a funny amount to get the app to know how to use the scope (That in your case, South hemisphere) I really like to see how you want to see that dim alpha centurion in first place.
Both can be roughly pointed to the either pole (Accordingly) and have good result when you use wide angle lens so don't make too much fuss about perfect polar alignment.
- Vixen has 2 AA and iOptron 4AA batteries. So guess which one will last longer, which one runs cheaper.
- Change of battery in iOptron is PAIN IN THE REAR END, Big Time... I have never had any device to have worst battery compartment than iOptron and I really hope iOptron addresses that in next version of this nice product (In every other way nice).
- iOptron has the equatorial wedge and adjustment built in, with Vixen you need a second ballhead (Functional and cheap ones on ebay are about $25) or a second small tripod head for this purpose.
- iOptron makes funny noise when at work and Vixen is almost silent.
- Vixen is about half the weight of iOptron so guess which one will cause less neck pain on a long hike to your shooting location.
- iOptron has excellent customer service and through this forum I have witnessed several astonishing nice after sale service to the owners that is one reason I may want a product in first place. I have never had any problem to call Vixen to figure out how they perform so no comment here.
- Vixen seems to be discontinued with no replacement at this level so good luck finding one ( Telescope.net has none but Amazon or B&H still have some (Not sure the customer returned items put back into new box or left overs from last supply.)
Bottom line, you will be happy with either and don't be surprise to see they can even move your heavier lenses and camera bodies than what you though. My only problem with heavier camera and lenses are inability of the tripod to hold them steadily and problems with maneuvering specially to point a long lens right up toward Zenith, rather than trackers ability to move correctly by the tracker's motor. My limit is 500mm mirror lenses or 300mm F2.8
For less money and free scope and no need to have second head for equatorial adjustment, you may want to go with iOptron. I like slick and would go with Vixen even if I am to choose again and would want iOptron address the crazy battery compartment before I ever buy from them again.