Unless you have decent monitors and plan to mix & release product yourself I personally wouldn't get all concerned over the converter quality on most modern midrange units. Most of them are quite decent now days. However, it's not all about specs on paper. The overall I/O circuitry plays a role beyond just converters. The only way to know is to do some research online and then listen to the top candidates for yourself. And buy from someplace you can return the device if you aren't happy.
What MonkeyMan pointed out about the harsh highs is something that can, to some people, be annoying when using a device with sub-par I/O logic. High gain guitar tracks can result in more of a "scratchy" harmonic content instead of "sweet".
Sonic