Let's say 24 seconds and forget the casting time for a bit:
Troubadour: generates 8 phrases --> can cast 2 skeleton invocations that cost 4 each --> 6 skeletons
Beckoner: generates 4 phrases --> can cast 1 skeleton invocation that costs 4 --> 6 skeletons
So in 24 secs the troubadour summons the same amount of summons - but they are sturdier and bigger. At the same time he can produce a skeleton every 3 seconds whith "Many Lives Pass By" while the Beckoner can only produce eone every 6 seconds:
So in 24 seconds the Troubadour summons 12 skeletons while the Beckoner summons 9. 6 of those are whimpy.
The advantage of the Beckoner is that all 6 come out at once and do damage right away - and that you can profit from killed summons. Which is easier if they die faster.