Which society was freer, you think, the Soviet Union's or apartheid South Africa's?
Hmmm...well that a difficult question. Granting limited "democractic freedoms" to only 20% of the population at most is hardly limited "democracy", it is rule by a few, a minority and could even be termed an "oligarchy" if one looks more at what each word means, especially when those freedoms are race-based. True the whites could vote between a plurality of parties, but most of those parties spouted the same message and only one party really mattered from 1948-1994 and that was the National Party...kinda makes one think of the PRI in Mexico from 1921 onwards...
Also the USSR never restricted the basic reproductive freedom of its people. You could have as many children as you wanted with whomever you liked (so long as the person was of the opposite sex and not a close relation). In South Africa you could be jailed for having sexual relations with persons of a different race.
Apart from that, the idea of communists and terrorists infiltrating the Republic of South Africa directly from Angola is geographically impossible since they would first have to pass through South African occupied (after 1969 according the UN) South-West Africa/Namibia. South-West Africa may (or may not) be considered RSA territory (just as one may or may not consider Palestinian territories to be sovereign Israeli territroy). If one considers then ANC and SWAPO to be communist threats to South Africa just because the MPLA may have sympathized with them is stretching it. The ANC arriving in South Africa through Mozambique and traversing Rhodesia and South-West Africa is more probable. Also I would like to refer people to this site :
http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB67/
which details Cuban involvement in Africa (the poor Cubans being demonized by everyone as usual, even though they were aiding Angolan independence movements (albeit leftist leaning ones)) and that the Soviets actually mistrusted the MPLA (so ideas of a Soviet supported monolith of African communist movements just seething to jump into South Africa and under the direct control of the USSR are...well...somewhat exaggerated)- see Document 3 on the website.
see also :
http://reference.allrefer.com/country-guide-study/angola/angola47.html
which interesting has the initial South African invasion force being made up mostly of Angolans...
At any rate, the question here is a South African occupation of Angola. Would it work? For a short time maybe, but I rather doubt the majority of Angolans will be entirely pleased with having the Portuguese leave and having Suid Afrika take their place as their masters. Most likely if UNITA is placed in power by South Africa and is perceived to be controlled by apartheid South Africa, the approximately 40% popular support they had will probably vanish in a few years and South Africa will face rebellion all over Angola by a reinvigorated MPLA which probably now has made a temporary alliance with the FNLA and what sections of UNITA dislike the South African influence.