The only difference between a keggle and brewpot is price and the amount of work you want to put into it, really. With a brewpot everything is ready to go as is, whereas with keggles you will probably have to cut the top off and put in fittings if you want. However, the keggle is most likely way cheaper to make. So as far as for better/easier/more efficient, there's no difference at all.
I use a keggle for my HLT and a megapot for the main boil. I find that the megapot boils the same amount of liquid much faster than the keggle with the same burner. However, I was able for find a used keggle ready-to-go for $50, while the pot set me back 5 times that much. Personally, I like the setup, as I am more concerned about my wort boil times than my HLT water heating time.
There's no harm in getting a keggle to start with. If you decide you don't like using it as a boil kettle, just use it for your HLT instead!
I use a converted keg and for the most part I am happy with it. The one main drawback that I have noticed is that unless you want to loose out on a gallon of wort, you can't really do a whirlpool. If I used an immersion chiller I would love to leave the bulk of the hops and the coldbreak back in the kettle instead of in the fermenter and this requires a flat bottom kettle. I use a plate chiller so the cold break is in the fermenter either way but leaving the tiny hop particles out would be great. I use a hop bag and a screen around my collection tube but one more "filter" would be great. Along the same note, in my boil kettle I have a wort collection down tube that comes out into the center of the keg to get the wort out of the bottom of the "cone". It is not that big of a deal but it is one more thing in the way of cleaning. If your boil kettle has flat bottom your wort collection down spout does not need to protrude further than an inch or two from the kettle wall.
If I had the money I would have gone with a brew kettle. I have seen ones that are smaller in diameter than the keg. When doing a 5 gallon batch in a keggle you don't have much height of liquid, this means my immersion chiller is less efficient than it could be in a skinnier brew kettle. If I were doing it over again I would build my own immersion chiller and I could somewhat compensate for this.
I've had a cut keggle in my backyard for over 2years now. I've never used it, I really like my 10gallon pot so I probably never will...My pot is really wide and low and it fits nicely underneath stuff whereas the keggle is HUGE.