OTA is basically an Opamp with high output impedance, meaning no output buffer.
Advantages: Generally greater bandwidth than opamps.
Disadvantages: Cannot use low impedance loads. Generally OTAs are used only for capacitive loads, since DC current is zero.
A MOSFET's gate for example is a capacitive load, so you can use OTAS if you're gonna connect it to MOSFET's.
Opamps are more generic, they're good for most applications that do not require large bandwidths.