The key properties for a good roasting pan are:
Since they are rarely used on the stove top, except briefly to deglaze, their heat conductivity is not really a feature that matters very much at all. Similarly, their non-stick properties are not normally important, because you want fond development to provide the basis of a good sauce.
Neither of the two choices you enumerate is ideal based on these criteria:
- Cast iron would be far from ideal simply because a pan large enough to hold a good sized roast or bird would be prohibitively heavy.
- Layered construction pans are very expensive, and provide little benefit, although they do perform well (but not better than alternatives)
Many cooks, both home and professional, including myself, don't actually use specialized roasting pans. Most kitchens contain at least three other pans that will serve the purpose just as well, without an additional expense at all:
- Sheet pans are ideal for roasting, and are inexpensive and useful for many other tasks.
- A skillet or saute pan can be used for roasting smaller cuts (and some big ones if you have a larger pan), and also is ideal for any searing or deglazing
- A casserole or lasagna pan can be used for roasting
For this reason, I recommend that you don't invest in a dedicated roasting pan. Instead, look to what more general purpose pans you have in your kitchen, and use one of them. If you don't have one, I suggest starting with sheet pans.
While I do own an enameled roasting pan, I almost never use it. My first choice is a sheet pan, or one of my other skillets.