Hey Carey.
Okay, first of all, the weight of the sword isn't that much of an issue, or at least it isn't if the sword is well made. All swords that are accurately-made enough to claim the name of a sword will weigh around 2-3 lbs. Also, again if they're accurately-made, they will be properly balanced so that the width of the blade is not much of an issue. There will be a slight difference in balance between a type XII and the other two, but more on that in a moment. The main issue, if you're considering types XII, XVI and XVIII, is what these blade-types are meant to do.
A sword with a type XII blade comes from a period when knights were well-armored with mail, and this blade shape is designed to combat this. The blade has enough width and flex to be an excellent cutting blade, but the point is more acute than the knightly sword had been previously, to assist in thrusting. The idea is to drive the point through the rings of the mail.
Type XVI comes from a period when plate armor was on the rise, and type XVIII comes from when the use of plate armor was in full swing. A sword cannot cut through plate armor, so more emphasis was placed on the thrust in the blade shape. Here the idea is to drive the point in between the openings that have to be in armor to allow for movement. Hence the more acute points on each of these types. A type XVI will have a bit more flex in the blade than a type XVIII, but might be slightly stiffer than a type XII. The fuller is not really meant to widen the blade, but more to lighten it. Type XVIII will be a little stiffer than both the others, because it is meant to thrust more. Another difference between XVIII and XVI is that a type XVIII will often have a hollow-ground blade. This enables it to have a thicker spine that gives it greater rigidity for a thrust, yet still enables it to be light and fast, as well as retain a good edge for cutting.
Now, about the handling. As I just mentioned, any sword that's well-made will be balanced properly so that it's not hard to handle. There will be some difference, though, between a type XII and the other two. Types XVI and XVIII will be balanced a bit more toward the hilt, as they are meant to thrust more than type XII. Type XII will be balanced more toward the blade. But any of these blade types will serve well as a cutting sword.
As for these types vs. a more leaf-shaped blade, we're getting more into the realm of speculation. As I mentioned before, these blade types were developed in response to developments in armor. If armor had not developed as it had, maybe we would have seen leaf blades of that length in the Middle Ages. But as far as I know, you don't see leaf-shaped blades much past the Roman Republic, and all of these were short swords. For shearing cuts with longer blades, most seem to have preferred a sword with straight edges. The leaf shape is, I think, meant to increase cutting power in a shorter sword by putting more width and mass at the point where the blade makes contact with its target. But in a longer sword, this is made up for by the length of the blade, so the extra width is unnecessary.
Well, I hope that helps you in choosing. Basically, it boils down to what you want the sword to be able to do. I would recommend Albion, if you can afford them: (
http://www.albion-swords.com) Their swords are well-made and handle really well.