It depends on who you fly with on which routes and which class. I dont know about SAA, but I will compare ET to TK here:

Advantage with TK Miles and Smiles is that you only need 40,000 to qualify for gold (Elite), which is valid for 2 years. And then soft landing to Silver. Also, it is easy to obtain Elite Plus (80,000 miles) which as far as I know has soft landing to Elite. That means 2+2 years of Star Alliance Gold. Also, note that TK does not have a fixed accrual dates that follows the calendar year. This means as long as you get 40,000 status miles in a year (could be April to March, for instance) you will be upgraded to Elite member.
Also, as Elite Plus you can re qualify for another 2 years by obtaining 40,000 status miles within first year.
Drawbacks: 0 mileage accrual on e.g. Ethiopian Business class P fares. Difficult to redeem miles on anything but Turkish. Very little award availability in high season/popular routes unless you opt for "Seat Guaranteed" which is very costly.

ET's Shebamiles:
As far as I know, you only accrue in a calendar year. So your mileage qualifications have to be Jan-Dec. Means it makes little sense to apply mid-year. All levels except Platinum are only valid for 1 year. As far as I know no soft landing (Correct me if I am wrong). Also, extremely difficult to redeem miles if you aren't either in physically in Ethiopia or on good terms with your local ET sales office.
Advantages: You accrue miles on all ET fares including their P-business class fares. Easy to get award upgrade availability on ET's flights, even the most busy flights to US/EU in summer period. However when upgraded it is per flight, not per leg. E.g. you can upgrade from ADD-IAD from Y to J for around 35,000 miles if I am not mistaken.
If you fly ET a lot, Shebamiles is the way to go. Especially if you fly ET business class.
If not, go with TK.

I personally went with TK miles and smiles, but since I fly a lot of ET nowadays I am considering changing. Also, until very recently all ET Y-fares credited 100% to TK. That changed, unfortunately.