Whilst I have and use an eye-level prism as a kind of default setting on my Sq-Ai, I have and use a wlf as well. I guess you could sum up the reasons why I use a wlf as follows.
Sometimes I want to shoot with the camera low or high. The combination of an eye level prism with a wlf lets me choose the viewpoint thats right - or, if I'm pointing the camera sharply up , to make it easier on the neck.
Sometimes I want to use the extra magnification in the wlf's magnifier to tune composition or focus.
Sometimes I want to see the elements of the composition in more abstract terms and having the image laterally reversed helps this.
It costs me little in money or weight to achieve these flexibilities. I could achieve some of them but not all by using two prisms but that costs and weighs more.
I use the ttl metering of my prism only as a sense check on my hand-held. My routine involves setting composition before I meter, and I don't want to then lose the composition whilst I move a prism around my composition to meter. I also want to use an exposure routine that translates from system to system, and taking multiple spot readings from around the frame with a hand-held spotmeter does that. I find the metering of my AE prism S to be pretty primitive; its an overall average meter, and that isn't how I want to control exposure for slide film. There is a later/more expensive version that spot-meters and though I haven't used it I doubt whether it can challenge the precision of a 1 deg. spot. I think it would be rare for me to actually set the exact exposure suggested by my prism's meter, but its good enough to remind me that I've changed backs for a two stop faster film and haven't reset the hand-held!
Incidentally most times you buy a Bronica, even on eBay, there's a wlf with the basic body/lens/back kit. There doesn't seem to be much merit in buying as separate components unless you really don't want an 80mm lens. Your real decision might well be "is it worthwhile buying a prism ?" rather then "which of these two mechanisms should I prefer?"