I would look up prices on the internet for the 2010 Triple Dolce Elite. I didn't look far to find that Bicycle Alley (in MA) currently has a 2010 Triple Dolce Elite on sale for $599.99. I would definitely show print out this online price to show your LBS. You never know if they are willing to price compete.
I did try riding with a tag-a-long once on my carbon road bike...I was worried that I would crack my seat post. So, I would say that if I had the Aluminum Dolce, I would definitely not worry about that. Also, since you have a triple on the 2010 Dolce Elite, I think that is great gearing to pull your tag-a-long
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2012 Specialized Amira Elite, upgraded carbon handle bars, Jett saddle 143mm switched to 145mm 2012 Selle Italia Max SLR Gel Flow saddle
2010 Santa Cruz Juliana with R kit and Crampon pedals
2011 Specialized Ariel Sport,suspension post,Serfas Rx Women's Microfiber saddle (sold)