From my experience, it's hard to beat the converted keg for a boil kettle.
1) They are cheaper than flat bottomed pots. A Universal Kettle from Sabco is just under $100. The flat bottomed pots of similar size are almost double. Sometimes the flat bottomed pots come with a ball valve, but it's usually brass and worth about $10 at Home Depot.
2) The Universal Kettle has 2 ports in it. One is a full coupling so that a valve can be attached on the exterior and a dip tube or kettle screen can be installed on the interior. The second port is a half coupling and can be used for a Sight Tube (very handy) or thermometer or both. Flat bottomed pots typically have one port, and it is a half coupling for attaching a ball valve only. I've had dozens of customers wanting to install a kettle screen into their flat bottomed pot, only to find out that the half coupling doesn't allow it. Many have opted to install a weldless fitting, adding to the overall cost of the their already expensive kettle.
3) The converted keg is just about indestructable. The skirts on the top and bottom make the keg very stiff. The flat bottomed pots, although durable, will not take the kind of punishment that a converted keg will. Try laying your kettle on it's side and standing on it. Two or three people could stand on a keg laying on it's side with no ill effects, a flat bottom pot will collapse without too much effort.
So, the final verdict is: cheaper, more flexiblity with add ons, and more durable. The converted keg wins.
Prosit!