The Synapse has relaxed geometry. The Supersix is more aggressive. I looked at the Ultegra in a 52 frame today. Weighs 16.1 lbs without pedals. Light as a feather. Tomorrow I'm going to check out a bike store that also has the Synapse and the Trek 5.2. Don't forget to compare apples to apples; compare the weights with the gearing you want. I don't know what the stock gearing on those bikes is, but I would be looking for a bike that is close to the gearing you want and won't require a lot of parts replacement. Have LBS weigh the bikes. The wheels on bikes, even high end ones, usually are not the best. You should ask your LBS about getting credit for the stock wheels and upgrading. Depending on your budget, a cheaper bike with wheel upgrade might help with your weight requirement.