Proximity probes provide very strong relative displacement outputs at low frequencies. However, they are difficult to install and, due to filtering in their electronics, give very little information at higher frequencies.
The electrodyamic velocity sensors provide very strong absolute velocity measurements at mid-band frequencies. However, they are nonlinear at frequencies below 10 Hz (600 cpm) and contain moving parts that can wear and fail. Their useful frequency range is, typically, 10 Hz to 1,000 Hz.
Piezoelectric sensors provide strong absolute acceleration signals over a very wide frequency range. They are extremely rugged, easy to install, and can provide a variety of outputs depending upon the application.