I have both, so I ran a quick test for you.

Firstly, here's the original DNG. It's a snap of mine, converted from Nikon NEF by Ligtroom, no editing.

I then exported it with Ligtroom, same size, no sharpening, 100% JPEG quality, and no edits.

(You can download it from here for pixel-peeping)

Then I converted the DNG through ImageMagick. Again, no edits, 100% quality:

(Here's the downloadable location)

You be the judge which gives the better export. However, note the following:

• The Lightroom version has been modified. Not only is the exposure different, but it's very slightly warped relative to the ImageMagick version. I think there might be some lens correction going on. If there is, I can't find it; all my "Develop" settings are at default.
• The DNG is 8.6MB. Ligtroom's JPEG is 7.3MB: not a very significant compression, but that's what you get at 100% quality. Imagemagick gave me a 14.2MB file.

Here's my take-away from this experiment: It doesn't matter.

• You're going to want to edit your photos. Your image editing will probably affect your results more than your choice of JPEG conversion tool ever will.
• You'll want to increase your JPEG compression by reducing the quality setting. Otherwise, you'll get files with barely any compression at all; at that rate, why not go TIFF or PNG? By increasing the compression, you're going to lose quality, and it will probably be more significant than the mere choice of tool.
  • This is especially significant with ImageMagick giving the wacky larger results than the DNG; it blows that option out of the water.
• How big of a deal are the quality differences anyway? You'll have to really pixel peep at full size to see any changes. If you get obvious visual differences (like my exposure shift), the differences due to conversion will be insignificant.