I think you also posted in Campy forum also asking about Tiagra, which is a Shimano group - right? Anyways, the groupo (Tiagra) will be workable, but nothing close to 105 or Ultegra. If you can spring for something with 105, then definitely do it, otherwise plan on upgrading when funds allow. I've ridden both groups and it's definitely a big step up to 105, imho.
The frame, Synapse Carbon, is a superbe frame and will be comfortable, not twitchy, and great no matter what group you have or eventually put on it. This is the frame ridden on the cobbles of Paris-Roubaix by Liquigas (I think) last year. The "6" comes with Tiagra I see, so go for that setup now and see what you may want to upgrade to in the future component wise if you can't spring for the 105 equipped "5".
$1800 MSRP? That seems pretty pricy for any bike with Sora/Tiagra components. I got an '08 Synapse Carbon 3 on closeout a couple months back for just under $1700 (full Ultegra components, Mavic Aksium wheels). Any chance on finding a closeout of last year's model?
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