This is one of those times where I wish the stepmania compatibility chart from the old site was still around where users report if a pad or controller works or not.
Not all USB dance pads are compatible. For example the xbox360 dance pads for DDR universe does not allow you to hold opposite direction arrows unless load a
driver to remap keys and correct the issue.
Soft pads are a good introduction pad to Stepmania because they're so cheap. Most you will find are for playstation 2 which is okay. There are such adapters as Playstation2 to USB converters which do work on PC's which tend to work alright. I can't say those adapters are the greatest in the world because depending on the USB bus you can get some latency with them.
If you do dive into the world of hard-pads. The only thing I would honestly warn anyone against is stay away from Ebay hard-pads shipped from china - The price tag will catch your eye with free shipping and all at a fraction of the price of a GOOD hard-pad. And they might even look nice. But due to poor plywood builds they will warp and break on you in a months time.