First, a 'cement' mixer is an incorrect term used by people who are ignorant about concrete. They also tend to say things like 'cement countertops' and 'cement driveway'. If I feel snarky, I ask them if they make their sandwiches with two pieces of flour.
A drum mixer is for traditional, gravel-based concrete. For concrete to mix, cement particles must be subjected to high shear forces, and gravel in a rotating drum does that. But a drum mixer is pretty much useless for a sand mix, it will give you the same quality of concrete as mixing it in a wheelbarrow with a shovel.
Mortar is concrete made with sand only, no gravel. A mortar mixer must provide shear forces some other way, so the drum is stationary, and there are rotating paddles on a shaft that do the mixing. Usually, the paddles have rubber tips that scrape the walls of the mixer. If you put gravel in a mortar mixer, say goodbye to the drum. You can have horizontal shaft mortar mixers (e.g. Stone) or vertical shaft mortar mixers (e.g. Imer 360), and it's really a matter of preference. The size of the motor or engine makes all the difference. People who have vertical shaft mortar mixers will disagree with me on this one, they swear by their Imers for GFRC mixes.
Personally, I find a good handheld mixer with an egg beater paddle is very effective for sand mixes, and it's about 1/10th of the cost.
Thank you Alla and Mark.. now i am enlightened! :)
Thanks to Alla and Mark for the explanations. I'm a little more qualified today. I've picked up a tidbit from both of you.
Alla, I'm afraid the terminology is an uphill battle for you. Around here anyway, even the masons call a rotating drum a cement mixer or cement truck.
When Alla gets snarky you better watch out.