A good starting point is to measure the width of your shoulders.
From there, the you can determined tradeoffs.
Go narrower for more aero
Go wider to open your chest more
2010 Specialized Secteur Elite with the RBR not-approved Mavic Ksyrium Elite wheelset
2009 Specialized Rockhopper Comp 29er upgraded to SRAM X9 with 1x10 and Stan's Arch EX rims and tubeless.
2013 Cannondale CAADX-6 Tiagra with 9-speed SLX RD to run 11-32T cassette and Stan's Alpha 400 rims and tubeless
Some Burton snowboard setups, one with stiff ol' Camber and one with Rocker-Camber-Rocker
If you get different bars make sure that you know how they're measured. Are the bars that you're using measured center to center or outer edge to outer edge? Are the bars that you're interested in measured in the same fashion.
42 center to center are pretty much 44 outer to outer.
If the 42 doesn't feel right then I would have no hesitation about going up or down a size (depending). All fit and measurements that I've had come out as 42 (c-c) being the right size. I hate it; 40 c-c feels right for me and this is what I happily use.