There's a bit of a difference on the front, too. The Ion has a field of view of up to 170 degrees depending on your recording resolution. The MHD's lens is 120 degrees, which is still plenty wide, and is actually about the same as the Ion's when recording in 1080p.
The two movie resolution options are 1080p and 720p (MOV format), easily selected with the switch on back. Just slide the big switch on top forward and the camera turns on and starts recording. If you want to capture photos, just press the power button at the front of the camera and press it again to take a photo.
A small application for Windows or Mac is stored on your microSD card (cards of up to 32GB are supported) when inserted in the camera. Launch it when the camera is connected to a computer and you can make changes to a handful of the camera's settings (this can also be done through an onscreen interface when the camera is attached to a display via HDMI). This includes changing photo resolution and shoot mode: single image, three-shot burst, or interval shooting that captures a photo every 5, 10, 30, or 60 seconds.
The MHD also has a built-in gyroscopic sensor that detects the camera's orientation, allowing you to mount the camera upside down without needing to rotate video 180 degrees with editing software. The gyroscope doesn't adjust for other positions, though, so if you mount the camera on its side or at any other angle, don't expect your image to level out.
Charging the built-in battery is done via USB -- plugged into either a computer or wall adapter (not included) -- and takes about 2 hours to get a full charge. You can record up to 2.5 hours of 1080p video on a single charge, which is pretty good, but once it's empty, it's empty; the battery cannot be removed or replaced. Obviously, it can be recharged.
If the MHD has a weak spot, it's video quality, falling into the "good enough" range for its price. If the most you want from the camera is to record clips in good light for posting to sharing sites and viewing on small-screen mobile devices, the MHD is sufficient.
At small sizes, things look reasonably sharp and detailed. However, you will still see effects from rolling shutter like wobble when the camera is vibrating from movement. Colors are oversaturated and highlights blow out fairly easily, but that's typical for action cams. As is the chromatic aberration (purple fringing) around high-contrast subjects.
The bit rate for its 1080p video is low at approximately 10Mbps and it shows. It doesn't handle complex scenes or fast movement very well at all, showing a mush of artifacts that only get more visible the larger you view the video. Low-light video is decent, though the artifacts are joined by noise.
The audio, when not being blasted by wind or underwater, is good, and the mic is sensitive enough to clearly pick up voices at normal speaking levels (not shouting or whispering) up to a few feet in front of the camera. It sounds somewhat muffled, however, likely because of the waterproofing.
Photo quality is actually pretty good when you have plenty of light. Probably nothing you'd want to make large prints of, but for posting to Facebook, it's fine.
The Monoprice MHD Action Camera might not produce the best video, but the whole package is a very good value. It definitely makes it affordable to do a multicamera setup, and it's just inexpensive enough to make you want to impulse one to play around with it in and out of the water.