I'm trying to find some accurate numbers, but all I've managed to find so far was a debate between Robert Conquest (who I think is a Stalinist, but I may need some confirmation on that), and Steve Wheatcroft.
Can someone link me to some more accurate numbers, preferably including the number of people accused of sabotage, counter-revolutionary activity etc?
Victims of the Soviet Penal System in the Pre-War Years: A First Approach on the Basis of Archival Evidence
J. Arch Getty, Gábor T. Rittersporn and Viktor N. Zemskov
The American Historical Review
Vol. 98, No. 4 (Oct., 1993), pp. 1017-1049
http://www.jstor.org/discover/10.2307/2166597?uid=3739552&uid=2&uid=4&uid=3739256&sid=21104288695241
Robert Conquest isn't a Stalinist, he is/was (not sure if he is still publishing?) quite anti-Stalin. In his books The Great Terror and Harvest of Sorrow he was quite critical of people in the west (left wing or otherwise) who tended to ignore the nastier aspects of Soviet policy in favour of its supposed utopian goals.
A debate between Conquest and Wheatcroft wouldn't be so much a pro/anti-Stalin thing, but a debate between a 'hard numbers' approach (Wheatcroft) and estimates based on a mixture of official and unoffical sources (Conquest).
Robert Conquest is like an inverse-Stalinist. /u/audiored's suggestion is probably the best actual historical analysis. Conquest is mostly motivated by ideology.