I have a comparison of the Diana+ and Holga 120N posted here:
http://www.dianacamera.com/holga_vs_diana
The Diana+ is a more flexible camera, having more controls (3 f-stops + Pinhole aperture vs 1 aperture). The Diana+ also has the ability to change lenses. Currently, Lomography offers 4 additional lens.
The Holga, on the other hand, is a very solidly built camera, and while not being as flexible, is still a very capable camera. It will take photos that equal that of the Diana.
They both use 120 film, which can be more difficult to deal with, since there are fewer labs that can process it. Ask around at your local Walmart, Costco or other 1-hour labs. While they may not be able to process the on site, many of these labs will send the film out to be processed for you.
The next option is to find a local pro-lab. These are labs that specifically cater to professional photographers and will be able to process your film on site. As you might imagine, it can be a little pricey.
Another film processing option is to send out the film to be processed. Check out these processors.
You can often get your film scanned when it is processed, in order to upload your photos.
The the best of all option, process the film yourself. BW film processing and even color processing are not too difficult to learn and also not very expensive to get into.
If you want to get your images digitized, you can also get an inexpensive desktop scanner to scan them in yourself.
If you think that 120 film is too much trouble, you can try a 35mm camera
Which should you get? I usually like to recommend the Holga 120N or 120FN for beginners, but lately, I've been leaning towards the 35mm lo-fi cameras for beginners. Why? They are cheaper and simpler cameras. If you get one of these cameras, it keeps the cost of exploring lo-fi photography on the cheaper side. You won't get quite the same look with the 35mm film, but it is so much easier to learn with and it's lower cost of camera and film will help you figure out if this works for you. If it turns out that toy camera photography is your passion, then you can explore getting the larger format 120 cameras.
Take a look at one of these 35mm cameras:
Holga 135 or 135BC
Similar cheap lens and operation as the Holga 120 cameras.
1 aperture setting (yes, the camera has 2 settings, but they are identical), 2 shutter settings, N (1/100 sec) and B (Bulb setting)
tripod mount, cable release socket
If you want the corner vignetting like on the larger format camera, get the Holga 135BC.
Sample photos:
http://www.flickr.com/groups/holga135bc/
Superheadz Black Slim Devil
This is a clone based on the popular vintage Vivitar Ultra Wide and Slim. It has no exposure controls and no flash option, but with it's wider than normal lens (22mm) it is capable of taking some outstanding photos.
Sample photos:
There really isn't a good Flickr group for these cameras yet, so the sample photos are from the Vivitar UWS
http://www.flickr.com/groups/57074580@N00/
The Diana Mini
Supplier: Lomography.com for about $60
Lomography took the Diana camera, and shrunk it down to 35mm format. The Mini, while it lacks the interchangeable lenses of the full sized Lomography Diana+, is still a feature packed camera (at least compared to the Holga and Black Slim Devil). The frame format is unusual. They stuck with the square format of the Diana camera, but it is centered on a standard 35mm frame. The reason for floating the square on the 35mm frame is that be using a standard 35mm format, most 1-hour labs will be able to more easily deal with making scans or prints, since they are set up for regular 35mm film. You can also set the camera to half-frame 35mm, which is 2 vertical 24mm x 17mm images. This translates to 2 images on a standard 35mm frame. Currently a favorite of mine, it lives in my camera bag so it is handy where ever I go.
2 aperture settings, 2 shutter settings, N (1/100 sec) and B (Bulb setting)
tripod mount, cable release socket
This camera is unusual in that it offers square format,24x24mm on a standard 35mm frame (24x36mm). It also offers a half frame option, 17x24mm frames (it doubles the number of exposures you get on a roll of film)
Whichever camera you choose, remember that the point of lo-fi photography is to have fun and don't sweat the details!
For Holga tips, check out this website:
http://goholga.com/
Then there's my own website:
http://www.dianacamera.com/
I have lots of tips, tricks and photos taken with plastic cameras.
For how-to videos for Holgas and Diana cameras, check out my youtube channel:
http://www.youtube.com/kaituba