My vote goes to Dark Tranquillity. They are actually continuing to improve and grow in their genre, while In Flames is really going through the motions while trying to appeal to all the trendy metalcore/emo retard fans.
In Flames did have some great albums back in the day, like The Subterranean EP through Clayman. The rest of their albums go from just okay to a total disgrace, with especially Soundtrack to Your Escape and Come Clarity being absolutely HORRIBLE (their last album I actually didn't find to be all that bad, surprisingly). Dark Tranquillity, on the other hand, have never put out a bad album. Easy decision for me.