Those are 2 very different pens. The cp1 is a 'slimline' pen the section is 8.7mm and the barrel 9.4mm diameter.
So first thing is if you can write with a slim pen, some people can't.
Both pens have relatively heavy caps, so I don't post either pen, or the pen will feel tail heavy.
Interestingly, I got and use the TWSBI Eco daily, rather than buying a 580. Not that the 580 is not a good pen, but I liked simple looks and low price of the Eco better.
I suggest you think about an EF/XF nib. In college, I used a pair of Parker 45 with a F nib, equivalent to a Lamy or TWSBI EF nib. The F nib made the ink last longer and I could pack more writing on a page of paper than with a wider nib. The only issue is that you may have to smoothen the EF nib, to get it to write smoothly. Also EF nibs are more sensitive to the smoothness and quality (or lack of smoothness and quality) of the surface of the paper.
Edited by ac12, 26 March 2016 - 02:24.