Shimano (as well as Campy and SRAM) comes in several levels of quality/cost. Dura Ace is the most expensive, lightest, etc. (for Shimano). As you work down the list parts get cheaper, heavier, lack a few features, etc. It's probably worth the extra dough to look for Tiagra or 105 over Sora/2300. However, folks have generally been pleased with SRAM's entry level offering (APEX).
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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