I've used both keggle and 14 gallon flat bottom kettle and they both work the same on my burner. My 14 gallon kettle came with ball valve, sight glass and diverter plate and (importantly) tight fitting lid. much prefer brewing on the latter. Maybe a little more expensive but has lasted 10 years thus far.
Agree keggle route is cheaper, just be sure you find a decommissioned one.
I think you have a great point here for anyone looking at getting a new or upgraded BK. The flat bottom with a thicker sandwiched base type kettle is probably a better piece. You can more easily utilize a hops dam or a device like a hop blocker with it and kettle losses might tend to be less. That bowl shaped bottom on my 15 gal Keggle holds a lot of wort ( and trub ) and you have to tilt it to get the last of the wort. I have adjusted many of my recipes about 1 gallon larger in volume to compensate.
Or you can install a dip tube.
Mine plugged frequently so I quit using it. It also hits my IC and combined with my thermometer probe ti won't allow the IC to sit all the way down in the wort.
Occasionally I'll screw in a 1/2 copper elbow with male pipe adapter and that helps somewhat but it's still not ideal. The other dip tube I have is 1/2 inch stainless and runs out into the center of the keggle. Pellets plug it up the times I've used it.