Cast Aluminum Vs Cast iron Dutch Oven
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.