Sounds like you just need a good custom built frame...

_________________
"It never gets easier, you just go faster..." - Greg Lemond

"Because technology alone is a poor substitute for experience." - Richard Sachs

I might agree if I had a very long or short stem, huge spacer stack or an unusual amount of seat post showing. I don't to any of those and I could ride one size up and still could say no to those three.

_________________
-For my next trick I will set myself on fire!
-This board and the world in general would be a much nicer place if everyone could just take themselves a little less seriously.
-Sticky Fingers John Rockefeller
-First one over the cliff wins!

I always chose the biggest size that I was still comfortable with, but with the last frame that I received for long term test, I had to choose a smaller size. Why? The manufacturer uses size specific tube diameters and carbon layup for each size, and the XL I tried for a few days was way to stiff and harsh: it is probably designed for the 80-100 kg rider, and I only weight 69 kg @ 186 cm tall. I then tried the smaller (which I found out weights almost 100 gr less as opposed to the usual 30-40 gr/size increase!) was much more comfortable, the 20 mm longer seatpost also helps. The stack and reach differenc were corrected by a slightly longer stem and more saddle setback. So first time I ride a frame too small for me... and I actually like it!

Fit is my number one must on a frame. It effects comfort and handling. IMO a to small frame seldom has proper weight distribution. Proper geometry based on my size is not negotiable for me. I'd sooner wear knickers that are to small

_________________
WW Velocipedist Gargantuan

interesting comment peruffo, do you know if other / most manufacturers do the same? actually, are you sure about the one for your bike adopting different tubes for different sizes? I wouldn't expect that, at least one tube profile for each size. maybe there is a couple, one for smaller sizes, one for biggers.. whatever, would like to know for sure.
as for your comment regarding weight differences, I'd add that for very light carbon frames as SS evo or the R5, Ive seen 20-25 gr differences between sizes 56-54.. So for the OP (size 51 or smaller) the weight difference should be even less, defenetly not worth it - as one cm extra of stem (Ive measured for the 3T arx) adds about 6-8 gr. so if you need 2 more cm in your stem, and a few cm of extra seatpost, that is likely to balance out the weight savings on the smaller frame.

having said all that, I 100% agree that fitting is paramount, but in my case, I used to run a 58 caad and swapped it for a 56 and feel so much better, faster handling, etc, being 1,86m tall (most bike fits I did would put me in between sizes as well). so the only real piece of advice I have is the one we less want to hear: try both sizes, same wheels and everything else if possible

Tour Magazine tested a ton of frames in three different sizes and usually the smaller ones were stiffer, but there were exceptions. IIRC Trek and Parlee were among them, but it's been a while.

_________________
"Nothing compares to the simple pleasures of a bike ride," said John F. Kennedy, a man who had the pleasure of Marilyn Monroe.

Seat height and reach can usually be accounted for with a longer seat post or longer stem. And given most modern frames have begun to increase stack across the board for the more casual riders, a size small might allow you to achieve a more aggressive position. However, head tube length is your limiting factor with Cannondale (they are on the relatively short end of the spectrum), and you may end up using spacers to get a little more stack.

Either way, I don't buy the weight saving argument. I would imagine that a centimeter of a frame's tubeset is lighter than a centimeter of stem, spacer, or seat post.

and a small frame may be stiffer but the longer stem, seatpost will not be

_________________
WW Velocipedist Gargantuan