Yea it's a close call. But they're both an enormous improvement over this:
I think Alex Iglesias, PGI's 'mech artist (who's also done art for CGL's BattleTech) really focused on aspects from both the TRO art and the original miniature sculpt, like the flat glacis on the thighs, the extended vertical pelvic armor plates, and the shoulder pads. I think he also added some lineage to the Nova Cat omnimech by adding a bulge to the mid-torso. One detail I really like that he added was making the arms hexagonal omnipods like a few of the TRO 3050 heavy omnimechs. With the nature of visual MWO customization based on weapon loadouts, that feature really makes sense IMO.
That said, I like the MWO art more than the new RecGuide art, but both are major improvements to the original TRO and mini and I'm happy with both designs.