Hi,
well you should at least difference between the systems ;-)
There is a huge difference!

Engagement, fun and safety in my view, will be affected by the hardware in use such as: Quality and size of the video screen (bigger the better), I use a projection system for DDR and most games for that matter. The style of dance mat/pad, either softpad and hardpad will affect the safety factor and timing (slipping soft mats). I've, more than once tripped over my softpads (which gives a pretty good indication of how 'balance challenged' I am), I would assume that a hardpad is more secure?!

Hi Brett,
that highly depends on the player.

"Impact Pad" (only USB)
1-6 feet, recommended for absolute beginner and beginner
single: save (make sure it is flat on the floor)
double: the player has to be careful when he moves to the other pad

Standard Softpads
1-3 feet, only when you have no other solution
safety: slipping, wrinkle very fast
tip: use packet tape to fix it on the floor (first edges, then sides)

Hard pads:
depends on the height and how good you can feel the buttons
* as higher the pads are - more danger to fall, when you move to far
(double is very dangerous, because the risk to go to far is higher)
* when you can "feel" the buttons very good, it will be more save

Stepping:
On softpads stepping is better for your health
On hardpads you need good shoes and a "soft" stepping movements. Some kids do stomp and that is not good.

Seniors:
Safety bar is highly recomended.

Safetey is quite an issue here, and I'm using Konami DDR but should have been clear it's for a console, retail offering - i.e. the slippery fabric pads - the standard softpads - which are awful for durability as well.

Just to clear this page up a bit, this realtes to the PS2 version of Konami's retail DDR with the standard softpad supplied in the package

Machine Dance has a very high motivation. For myself it has one of the highest. The key is to have the music also on a CD. When I listen to the music, I get high interest to play it :-)
Stepmania has a "jukebox" and that is great. Just start it and after setting up you can start the game :-)

Machine Dance is quite dull. For myself it's not that appealing. This is reflected in my scores for DDR above which quantifies and explains my point of view.

some points for central information:
developer, publisher, website, system, age rating, software, hardware, both, own system, additional needed, ...