If I were you, I would hold off on selling the ST 1300 for now. Do yourself a favor... find a DL650 or a DL1000 and take it for a test ride. If you can, test ride both, maybe even back to back.
Why? Simple... you may already have what you really want, or you may realize that one of the DL's is your dream bike. But you can't figure that out from a bunch of internet posts, you have to ride them all to figure that out for yourself. I've never ridden an ST, but a good friend of mine has one. He went from a VTX 1800 to an ST 1300 and loves it. I still have my VTX 1300 and I now have a DL650 as well. I considered selling the VTX but it's just a completely different bike, built for a completely different purpose.
What I think you may find is that the ST really will be better for longer rides, especially at highway speeds, loaded with a passenger and luggage. My guess is that it will have better stability and just an overall better feel in that situation. However, when you ride a Vee or a Wee, you will have a smile on your face from ear to ear. These bikes really carve corners and perform rather well on dirt roads as well. I wouldn't take an ST where I'm now totally willing to take my Wee.
I'm keeping the VTX for two-up riding, the Wee is my commuter and fun bike. If you can afford to insure two bikes, consider that as a possible option. If not, at least ride the DL's before you sell your beloved ST. Seriously.
Good luck, and let us know what you decide to do!
2008 DL650 a.k.a. Road Warrior, member #16 of the Flat Black Club
2009 Aprilia Tuono 1000R a.k.a. Silver Thunder