Which had more effect on relations between the USA and the USSR in the late 1960's and early 1970's?

The latter--Nixon's initiatives leading to his visit to the USSR in 1972, and detente. The soviet invasion of Czechoslovakia in 1968 had little efect on relations, which had been almost consistently poor since 1945 anyway. The soviets had previously invaded Hungary in '56. Neither that nor the Czech affair in '68 lead to war or any big change either way. Of course one could argue that detente had little real effect either, since "a relaxation of tensions" didn't prevent the 1973 attack on Israeli occupied Golan and Sinai, by soviet equipped arab armies, the fall of Saigon in '75 to soviet equipped communist forces or the soviet invasion of Afghanistan in 1979.