Bike Size

Liam28

Looking for advice on new bike size, but not of the 'what size should I get' variety. I have done the size guides and measurements and can get a 56 or 54, so would like to hear what are the pros / cons of smaller or bigger frame size. I know smaller is lighter and more control.
I am getting a Trek Domane, am 176-177cm tall, and the Trek guide says the 56 is best: https://www.trekbikes.com/ie/en_IE/sizing/#sizing_table_bikes_road
Priority is endurance / comfort
Current bike is a CX 52cm.
I have tried both sizes in the shop and both seem fine. Most internet advice says if in doubt go for smaller frame, but why? And what are the advantages of the bigger option?

Buzwaldo

The reason for the general advice to go smaller if undecided is that it is easy to 'enlarge' a bike by putting a longer stem on it and raising the saddle a bit, but it's harder to make a bike smaller - putting on too short a stem is not ideal, can make it a bit twitchy.
I'm just about the same height as you (5'10" on a good day), and had the chance recently to pick up a second hand Domane in 54 cm version as a second bike. It's just right for me size-wise with a 110 stem, but everybody's proportions are different. (I've relatively short legs, longer back, for my height).
My primary bike is a canyon endurace in small, which is about 52 cm (changed to 120mm stem)
Both bikes are very comfortable, the trek possibly marginally more so, but the Canyon is easily a kg lighter, if not more (this is down to the model of bike rather than size). New Canyon also cheaper, but you don't have the assurance of dealing with a bricks & mortar shop.
Love both bikes, but if heading for the hills I prefer the canyon, but the Trek feels more 'planted' for a fast descent.
I'd say either size would probably be ok for you, but beware a shop persuading you that a particular size is right just cos they have it in stock.
Either way, enjoy.
Edit - just see you tried both sizes.

magicbastarder

one other rule of thumb i've heard that probably has a little truth and a little BS in it - is if you're between sizes, measure your arm span, and if it's greater than your height, go for the larger frame, and if it's less than your height, plump for the smaller frame.

Liam28

Thanks guys. I have read both these explanations. Btw the arm span measurement is known as your Ape Index;). Mine is much longer than my height which suggests the bigger frame.
And maybe the reason that bike fitters and the LBS recommends the smaller size is that you can always make it bigger, but you can't make a 56 into a 54. But I'm sure that both sizes could fit equally well with proper fitting, so I guess what I am asking is what is the difference on the road? Is it control, 'rideability', performance, acceleration, endurance/comfort? Is either one better uphill, downhill, flat speed, long spins? Or is there no noticeable difference once the fitting is done right?
I said I tried both sizes, but I just did a standover on the 56 and took the 54 out in the car park. A friend has a 52 which I took for a spin and it was too small.