It really boils down to batch size and boil times. Sorry for the pun!! I couldn't resist.
No down side to a keggle at all, other than the work involved in making it. The only thing I can say is. You have ask yourself how much wort you will be collecting? What is your boil time? and, What is your boil off rate?
I just upgraded from a keggle to a 20 gallon pot. I was doing mostly 10 gallon batches and if I wanted to boil for 90 minutes, I was pushing the limits of the keggle. I was on the brink of a boil over almost every session. Armed with only a squirt bottle. Sometimes if my efficiency was better than expected I'd collect more wort to hit my target numbers and guess what I'd was doing the squirt bottle jig again.
Just food for thought that will save you the headache and most importantly the cash. By the way were are you located? I've got a keggle sitting in my garage doing nothing.