Fran,
The advantage to using edge triggering is that you can have the external interrupt constantly active and it won't jump back to it until the incoming signal has gone high and then returned low again. With low LEVEL interrupt the program has to disable the interrupt and watch the interrupt pin until it returns to the high state to reactivate the interrupt or the interrupt would constantly return for as long as the low level persists.
In my case I am counting the number of clock overflows to determine the RPM of a motor. I am using a HALL sensor which goes low when a magnet passes close to it and returns high when the magnet is past. In this situation there is only one falling edge for each revolution. So using the edge triggered mode should allow me to have the interrupt active all the time.
However I have found that there is a problem with the Intel 8751H and the Dallas Semi 87C520 when processing these edge triggered external interrupts.
If I choose edge triggering and leave the interrupt active, the interrupt returns constantly until the external interrupt pin goes high.
That took me a while to figure out but I was dismayed to see that when I switched from Intel to Dallas Semi the problem still persists.
Did anyone else notice this?

I hope this sheds some light on your question.

Buck Stein