Christopher Hitchens, former Trotskyist and Robert Service, author of a recent biography of Trotsky, brilliantly address this question in this Uncommon Knowledge discussion beginning at 12:54. read more
However, Trotsky was not a cautious figure like Stalin and would very likely have started a war in Europe in the 1920s or 30s as he would have attempted (and failed) to export the communist revolution to Germany - although that event in turn would have preempted the rise of the Nazis. read more
The odds of a Trotsky USSR starting or provoking another large war are quite high. If that doesn't happen, Nazi Germany is very likely to destroy the USSR and exterminate/enslave Russia. Trotsky would not have industrialized like Stalin did (including huge heavy industry far from borders) because that wasn't his worldview. read more
Trotsky himself wouldn't have been able to change the material circumstances of the Soviet Union, and it was these material circumstances combined with Stalin's policies that led to the degeneration of the Soviet union into what I'd call a 'State Capitalist' system. read more