After having played a few games of Warmachine, I think I have a fairly good grasp on the mechanics and the rules involved. For most, it always seems to be Warmachine/Hordes vs. Warhammer 40k, which frankly is somewhat silly. They are two COMPLETELY different games. Perhaps I am misinterpreting the forums and flame wars on both sides, but I seem to get that vibe a lot. After playing both, I wanted to discuss my pleasure and displeasure with each, in order to inform someone looking into each. I advocate playing either! They are both fun and tactical table-top games. The similarities between them are loosely comparable, but they are very divergent games.
Warmachine/Hordes- The perks of this game are fairly obvious. Fewer models means a smaller initial and continued investment in the game. The single unit system is fairly interesting, and the rules use a lot more "realism" to make the game more tactical. The casters allow for a wide variety of play-styles and army compositions with just one faction. The factions are well balanced overall, although some factions have far superior units.
The main issues I have with the game overall are the scale and the models. While fewer models does mean less money, the grand scale strategy is far more interesting in my opinion. In addition, I find the Privateer Press models not nearly as impressive as 40k. The IP is also fairly weak, and it has little depth. Legitimately, the only reason I have my Scyrah is due my appreciation for the faction aesthetic and my slight hard-on for the elves.
Warhammer 40k- Certainly, 40k has a much larger audience to participate with, and it has some of the best IP in the Science Fiction world. It has a lot of awesome models, tons of different factions to play, and a rather large variety within the codexes. The main issue: Balance and cost. There are some factions that far outclass others, and the models are incredibly expensive. There are some wonky rules involved, but that is usually house ruled anyway.
Overall, I still really enjoy both games. They each have their own perks and downsides, but it seems silly to insist on one being better than the other. They are two totally different games, and you are allowed to play and enjoy both.