The biggest practical differences are going to be:
Nothing sticks to a well-seasoned cast iron pan. Everything sticks to cast aluminum, even the brands that call themselves non-stick. (I know because I've tried a few of them.)
While it's true the cast iron can crack, rust or chip if not properly cared for, aluminum is reactive and scratches easily, and even if you get one of the newer anodized ones, you lose the benefits of that as soon as the surface starts to chip and peel (which it will). Cast iron will last much longer when properly maintained; I've seen some that are decades old and heard about some that are more than a century old.
Aluminum has better conductivity and will, in theory, provide more even heat than cast iron. However, every cast aluminum vessel I've ever used has had major problems with hot spots. Basically, both are poor choices in this respect, except in the oven, where hot spots are essentially a non-issue; if your oven is like mine and the element turns on and off to maintain temperature, you'll get much better results with a cast iron dutch oven because it will hold its temperature while the element is off.
Cast iron also works great with induction cooktops, aluminum doesn't work at all. [Cast] Aluminum really works best on standard metal cooktop stoves, although it's okay for glass as well. I say okay because most glass cooktops tend to have elements that go on and off, so depending on your particular stove, this might lead to uneven heat when using any kind of aluminum.
Cast iron is heat-resistant. You mention the melting point, but it's more complicated than that; make sure you check the specifications on whatever cast aluminum you're thinking of buying, because a lot of it isn't even oven-safe (and if it is, it's only safe up to 400° F or so). The term "dutch oven" can be quite a misnomer for those pieces as they can easily warp or crack at high temperatures. Spun aluminum is obviously much worse but cast aluminum is only marginally better.
On the plus side, cast aluminum is obviously much lighter than cast iron and requires less care. Enameled cast iron is also easy to care for but is obviously much more expensive than both regular cast iron and cast aluminum.
In your case, it would seem that your two primary requirements (caramelizing sugar and roasting meat) are at odds with each other. For roasting, you want a very steady, even heat. For caramelizing sugar you need precise control, you need to be able to reduce the heat very quickly when you hit the melting point. Cast iron would be absolutely terrible for caramelization, but is a far better choice for roasting.
Personally, I use a regular (non-enameled) seasoned cast iron pot as a dutch oven and just use a small light stainless steel saucepan when I need to do something like caramelize sugar. I'd suggest you do the same, unless you're really low on space and genuinely need one piece of cookware to do it all.
Honestly, the only thing that cast aluminum really has going for it is its price. The ease of maintenance is overshadowed by the fact that the pieces don't tend to last that long, and everything else it's good at is handled equally well or better by stainless steel with an aluminum or copper core. The few pieces of aluminum (or cast aluminum) cookware I still own tend to sit at the bottom shelf at the very back and gather dust, but YMMV.
All my attempts to "clean and repair" a sticky-seasoning layer have ended badly as well. I know who already snickers in the background, but my opinion is: start clean. Give the pan a lye bath and a good scrub, then reseason with a better seasoning.
"The new hotness" of flaxseed oil goes back to the fact that it doesn't go rancid easily, without heat it just dries out making a film which can protect things like wood. I bought into it, and seasoned a pan with it, and wondered why all my seasoning tries were unsucessful (I stripped and reseasoned 3 times, I think). I came to blame it on the smooth forged iron.
Then I got Cookwise and read the chapter on fats. And surprise, what does it say? That PUFA stick to everything around. (Makes sense, if you stop to think about it - those unsaturated bonds are unstable, they itch to break apart). So, when I got my next iron pan, I seasoned it with three (very thin) layers of flaxseed oil (for a solid base which will stick to the pan and the finishing layer) and then two layers of lard. After the oven, I heated it from brown to almost-black on the stove (empty).
This seasoning turned out perfect. I made crepes on it, re-oiling once every 7-8 crepes only (no fat in the batter). The pan released them like a charm. The old flaxseed-seasoned pan still sits around with some carbonized matter burnt onto the seasoning, and waits for a lye-flaxseed-lard session.
Of course, I would still take some caution with new seasoning and oil well the first few applications (I only tried the crepes after I noticed that less problematic items work great). And if you tend to often fry with the fashinably-healthy nonsaturated vegetable oils, don't heat them too much, else they could bake in a sticky-seasoning layer onto the pan. In the worst case, if you do get sticky-oil (but not carbon) buildup, try adding a new lard layer before you strip-and-reseason.
Best Answer
I would not heat up a large cast iron press - too much to go wrong there, little benefit. You are not frying on the thing.
Clean any oil/rust that's on it, dry carefully, oil it with mineral oil or wax it with beeswax (or paraffin wax, but beeswax will probably stick better,) use it, clean it up well, dry very carefully, and re-oil/wax before storage.