White pine heartwood is moderately rot resistant. In the latest issue of Timber Framing, Jack Sobon indicated he used white pine for the sills of his new barn, os that is at least one positive endorsement.

My only experience with hemlock for sills is that my 1820 farm house sits on hemlock sills. They have held up fine, where kept dry. A couple of sections were not, and so had to be repaired. We replaced the rotted (and missing) sections with all-heart white oak.

My first outbuilding sits on white pine sills, but since then (addition and garage and the new workshop currently in progress) I've opted for all-heart white oak.

If your choice for continuous members is either sills or plates, I would opt for continuous plates. I believe if you have to scarf, it is better to scarf your sills rather than your plates. Of course, your architect or engineer will be the final word on this.

Cheers,
Dan