Ideally, we need both. A moderately sensitive charge accelerometer for low frequency and modal resonances, and a very sensitive ICP one for spot measurements.
Thanks to eBay, these accelerometers and a specialized amplifier (called "signal conditioner") can be had for very little money. They would probably be somewhat out of calibration, but 1) it's not prohibitively hard to do yourself and 2) what we are mostly interested in are differences
rather than absolute measurements — and an accelerometer that is slightly "off" would still highlight these differences perfectly.
I am using an Endevco Model 133
signal conditioner bought second-hand for about $250, an Endevco Picomin 22
charge accelerometer (about $100 used) and a high sensitivity Endevco 7250
ICP accelerometer. Equivalent models from PCB Piezotronics or Dytran are just as good. If you get the accelerometers on eBay make sure the cables are included. The connectors are decidedly non-standard and very expensive.