I'd probably vote for the new Tiagra as well. 4600 mimics 6600, which was an improved design over 6500. 10 vs. 9 isn't a huge deal, but the cranks/bottom bracket and the shape of the STI's were...
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"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur