You haven't said what size you need, but here is a nice one in my area: http://www.rodbikes.com/closeout.html Of the bikes you mentioned I would go with the Cannondale. The Tiagra is not the coolest, but in my experience it works well. My son has it on his Burley and it has never let him down. Besides, as Rudy has pointed out, if you are as serious as you seem to be you will be looking for a newer tandem in the near future.

Another excellent option is a new Cannondale RT2--if you can find one. I would check in at Tandems East for this enquiry. Should cost about $3500. While you are there check out the other new "old" stock as well.

In the end you should ride each of the prospective candidates and buy the one that feels the best. Low weight is cool, but is not the deal breaker it would be on a single.