I've shot for several years with both the Holga (actually, MANY types of Holga!) and the new Diana. Each camera has its strong points, and each has its faults. I think a summing up of each camera's specs and quirks is a good way for people to judge for themselves.
Has three aperture stops, one shutter speed plus bulb mode, plus a pinhole option. Going with the widest aperture makes for quite a narrow DOF and the in-focus "sweet spot" gets quite small at times. Viewfinder placement is directly above the lens, which needs to be compensated for slightly when shooting a close-up subject. Shutter is fairly sensitive and requires little effort to operate. Focus ring is on the front of the lens, not visible from photographer's viewpoint at the rear of the camera...which can lead to forgetting to reset focus when changing subjects (a case of "out of sight, out of mind"). Flimsy plastic construction--it feels like it would break easily if you dropped it onto a hard surface. It can be a bitch to load, especially if you're trying to do it quickly in the field. 4x4 image comes with a few light leaks and some vignetting. A fair array of accessory lenses and flash, of varying quality. Camera back has a sturdy slot-fed twist lock at the bottom. The camera takes it's own accessory flash only, unless you get an available hot-shoe adapter so that you can mount a standard hot-shoe flash.
Two aperture stops and one shutter speed plus bulb mode. More solid construction than the Diana, though I still wouldn't drop a Holga down the stairs. Viewfinder is in classic rangefinder position, which needs to be compensated for considerably when shooting a close-up subject. Simple shutter operation, but the shutter needs to be forced firmly to go off. Viewfinder is slightly deceptive: it actually covers only about 85-90% of the frame, which needs to be compensated for--this can be done by taking a full forward step before shooting. When the internal frame mask is removed, the Holga can produce sometimes spectacular light leaks and vignetting, which really show up in B&W. Easier to load than the Diana, though still not a cakewalk if you're in a hurry. Numerous accessory lenses and flashes--some good, some very good, some so-so. Lens is traditional focus, with easy to read symbols clearly visible from photog's POV. Lens is slightly wider-angle than the Diana's...a clear but not enormous difference. 6x6 image can go all the way to the edge of the film, sometimes through the film numbers. "Sweet spot" can be quite small, depending on the lens quality--another Holga quirk: uneven Chinese quality control, which many consider to be part of the camera's charm. Camera back can come off under duress, but this is easily remedied with a little tape. Comes with a standard hot-shoe, so any Holga or other "regular" flash can be put on the camera.
I personally prefer the Holga, as it is a somewhat more versatile camera due to it's numerous accessories, plus the fact that it's easier & quicker to load in the field than the Diana. I also prefer the sturdier feel of the Holga--I've literally climbed up and down cliffs with one around my shoulder and never felt the urge to worry about the camera. I would not feel the same way about the Diana. While I do like the Diana's 3 aperture stops, for me it's not quite enough to make it my #1 plastic camera...the Holga's vignetting and light leaks are what sells it for me. Hope this is helpful.