Mostly it just depends on what you like. I put 42 cm's on when I built out my Paramount frame. Never really liked them But that is what most say I should have measuring to the bony bump in my shoulder. My Tarmac came with 42 cm's too. And I've since noticed that on longer rides, my wrist will bend inwards toward the frame when in the drops long. And that starts making my wrist's numb.

I've been saying for the last six or so months that I was going to go back to 38 cm bars I have on my Raleigh. I don't remember having any issues with my wrists and hands to the extent I have on my Paramount and Tarmac when on longer than 2 hour rides. Maybe I'll make getting the new bar's a New Years resolution.

With wider bars some say you have better control and your get better breathing. Going narrower is supposedly easier on your back and shoulder muscles, though I haven't noticed that difference.

So unless you have to have that money for another set if you don't like them, I'd just try them out for a thousand or so miles and see what you think.