They're not identical - because the first is like having a zero-volt source between the two nets, and the second is like a voltage-controlled voltage source. So that means that in the first case current flows between P1 and T1, but it doesn't in the second case. You should never (well, almost never) put contribution statements within an if statement (the "almost" case is when you have a switch branch which switches from a voltage source to a current source - usually to model a switch opening or closing). Otherwise you end up with a contribution that isn't active all the time, and the node (or branch) ends up uncontrolled for some of the time, which presents a problem for the matrix solver, as it means you have an ill-conditioned matrix to solve. Regards, Andrew.
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