We have switched to 33P for cycle sequencing in our MJ Research PTC-100.
The company informed us several months ago, that with the thin-walled reaction
tubes used in the machine there are problems with 35S. They said that 35SdATP
reacts during temperature cycling to form a gas that can leak through the walls
of the tubes and make... well, a mess. Yes, its more expensive but I'd rather not
be breathing in my work! Hope this 2 cents helps...