the diana i have yields a 4x4 image on 120 film and my holga (from holgamods.com) yields a 6x6 image on 120 fim.
in my experience the diana images are really, really sharp at the center of the negative on a 10 x 10 print...and then make a smooth transition of being less and less sharp focus out to the edge of the negative.
i don't use either enough to give you a more comprehensive explanation of the good, the bad, and ugly of either one.
i generally get more printable (to me atleast) images from either ilford xp2 or kodak cn400. it seems both films have really, reallly w - i - d - e exposure latitude which is a good match for the sometimes slow/sometimes not-so-slow shutters of both cameras.
i bought an agfa ????? (looks & feels & sounds liker a diana) and have yet to run any film through it. i've been tempted by the agfa "clack" a simple 6 x9 .....tempting, but when would i use it ? if you would like i'll see what model it is when i get back in town. it may give the same results as the diana at a more reasonable price than the diana since it doesn't seem to have the same "cult" status or appeal.
there seem to be several variants of "th real diana" and lots of copies of it as , well...they seem to go for outrageous prices on ebay. you can find them at flea markets, antique shops, and thrift/resale shops also.
doi your self a favor and buy a diana first - mine had no light leaks and my holga leaks like a....you gotta tape most holgas up at the edges of the back and the film window. still, fun cameras to use and can give some surprisingly good images. " your actual mileage may very, blah, blah, blah...."
breathe, relax and enjoy!!
NEVER FORGET BESLAN