If you're prioritizing purchases of pans, Id put the stainless steel Belgique ahead of the cast iron, simply because there are many more uses there (at least with most of my recipes). But, this should be on the "second" list. Also, Lodge brand is one that is from America's oldest family-owned cookware foundry. Lodge has been forging cast-iron pots and pans since 1896. Preseasoned and ready to use, these heavyweight skillets heat evenly and won’t cool down when you add foods, making them ideal for searing and browning.
Preheating a cast iron skillet has a few invaluable benefits. A skillet's concentrated heat can be an asset for many baked goods. The heat provides that extra caramelization on the bottom of a tart shell, bar cookie; it helps develop the slightly crunchy crust on cornbread, the almost jam-like roasted fruit topping—once flipped—of an upside-down cake, and it puts bubbly blisters on your naan. A dry hot skillet gives you a blackening char; adding fat to the pan is the way to a nut-brown finish.
Placing a small amount of cooking fat down on the hot cast iron and waiting for its temperature to climb not only greases the pan for the optimal "sear," but also helps to build up the skillet's seasoning, which, in turn, protects the pan and increases its nonstick capability. Dave Arnold explains in his indispensable post "Heavy Metal: The Science of Cast Iron Cooking," published on his blog, Cooking Issues, that heat plus an unsaturated fat—of the kind found in some vegetable oils— yields a chemical reaction that changes the fat's molecules to form a polymerized coating that repels water, which makes it nonstick. A new skillet (and sometimes an old one) must be properly seasoned before it's used; when the pan's ready for work, by cooking on it with more fat, you will continue to build upon that layer and increase its hydrophobia.
...........Preheating: truth is, I don't usually have time to do all this babying, but I do wait for a pan to preheat at the temp I want. Charlotte Druckman* says because cast iron is slower to heat and does so unevenly, preheating, essential for searing, is also almost always recommended for frying, roasting, and baking. You can do this in your oven or, when you're doing a stovetop preparation, on a burner. Ideally, it should be done gradually. This is easy enough in your oven—just put the pan in there while you preheat the appliance.
It's a little more involved on the stovetop, but not much: Start the skillet over low heat, incrementally increasing that to whatever your desired cooking temperature is. Make sure you place the pan an inch or two off center and rotate it every couple of minutes, so it circles the heat source; this keeps the middle of the skillet from receiving all that direct heat and developing a hot spot. J. Kenji López-Alt recommends you give this a solid 10 minutes. If you think your pan is too hot for the pending task, lower the heat, and remember that cast iron is slower to register temperature adjustments.
*Charlotte Druckman from Stir, Sizzle, Bake: Recipes for Your Cast-Iron Skillet.
PURCHASE: a 12" lodge pan at Williams-Sonoma can be around $40, while Target has one for $20.