Cannondale Synapse vs CAAD8

Hello,
I am about to buy either a:
1. ($1350 CDN) 2015 Cannondale Synapse 105 with a 58cm frame (may be a size too big but haven't tested it yet)
Or a
2. ($1500 CDN) 2014 Synapse 105 that seems unreasonably priced but is my size at 56cm.
So, my question is, as a 5'11" guy with a 32'' inseam, could I make the 58cm option work and save money while getting a newer model or will I feel a big enough difference on the older, more expensive 56cm frame to justify the $150 difference. This will be my first road bike so it will be hard to judge which fits best after a short test ride so I want advice from experienced riders, like you. Cheers!
EDIT: How does the CAAD8 manage on rough roads? Is 105 noticeably better than Tiagra for beginners?
I am about to buy either a:
1. ($1350 CDN) 2015 Cannondale Synapse 105 with a 58cm frame (may be a size too big but haven't tested it yet)
Or a
2. ($1500 CDN) 2014 Synapse 105 that seems unreasonably priced but is my size at 56cm.
So, my question is, as a 5'11" guy with a 32'' inseam, could I make the 58cm option work and save money while getting a newer model or will I feel a big enough difference on the older, more expensive 56cm frame to justify the $150 difference. This will be my first road bike so it will be hard to judge which fits best after a short test ride so I want advice from experienced riders, like you. Cheers!
EDIT: How does the CAAD8 manage on rough roads? Is 105 noticeably better than Tiagra for beginners?
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You sound like a 56 but could be a 58 depending on exactly how your 5'11" is made up (longer/shorter legs, torso, arms, etc.).
Cannondale CAADX
Charge Plug 1