I raced the Dart 18 for 10 years (once with 300 other Dart 18s on a start line), many times against the Hobie 18.
The Hobie certainly is a lot heavier than the Dart. The Hobie points closer to the wind, having dagger-boards, except when the wind is force four or higher, then, the Dart can point as high as any cat (well almost).
There is more buoyancy in the bows of the Hobie, less likely to pitchpole. The Dart, with its smaller sail area, is easier to sail solo. There is not a lot of diference in top speed, the Dart, with its fine hulls, is a flier on a close reach in a force 4.
It probably will come down to crew weight, light crew - Dart 18, Heavy - Hobie 18. Cruising, with friends - Hobie 18. Speed when the wind is up - Dart 18.