Hi. First of all, in BJT, the carriers involved in amplifying or switch mechanism, are both negative e- and positive h+, in their two possible roles of minority and majority carriers.
In FET instead, only one of two possible carriers (of majority) constitutes the driving current of working.
Furthermore, with BJT is more simple to obtain a voltage amplification with low input impedance and low output impedance, by means of a little control current...
With FET instead, a current amplification (with a little voltage control) is more simple and the two impedances are both high.
FET, that can be used also as a resistor, is more fast as switch, but it is very electrically delicate in his operating modes.