I'd recommend Sarnia, especially if you are planning on bringing anything with you into the US. The Detroit/Windsor border crossings are notoriously more "by the book" and less lenient than any other US/Canada crossing, and the Ambassador Bridge is much worse than the Detroit/Windsor tunnel.
If going through Detroit/Windsor would save you significant time/driving (which I don't think it will) I would recommend the tunnel and avoiding the bridge if you could. You can still back up at the tunnel, but in my experience (I live in Windsor and work in Detroit, so I cross every business day) you're always better off at the tunnel, barring a big event in downtown Detroit or something odd like construction.