I have tried using both while fishing for Mahseer and the pro's and con's are like this.
Light spoon / plug / light Line- Long Cast -Fixed spool
- Hit a big fish &
Heavy spoon (3" to 5"), large plug/ Medium to heavy line - Medium cast - Fixed spool
& chances of loosing the fish due to line breaks
So unless i am fooling around, i just don't compromise on line BS and therefore use the multiplier.
The added advantage while fishing in a river is that you are invaribly lure fishing in fast water, so you don't have to cast very far, let the current do the work for you on heavy tackle.
You have to be very skilled with a fixed spool, and ready to swim/tread water if you have any chance of landing a biggie.
On the other hand with a multiplier you just adjust the casting drag properly (most of the Abu's below 7000 don't have one) and you can learn to control line flow while casting. After a while you don't even need to adjust the casting drag.
So i would say that depending on the water that you frequent, the fish therein, and also your style of fishing, you must select you rod and reel accordingly.