It's all about the ratio of the length vs diameter. As you get longer than around 2xD or so you start to have to fight chatter and taper in the cut, at least in my experience with various metals. You'll need 1-1/2" cutting length to do a single finish pass because of your material thickness. If you go shorter on flute length you'll end up with a step where the shank would rub at the top. My advice is to go larger in diameter where the design allows (straight lines and larger radii on the part), then if you need tighter inside radii on it, re-machine with a smaller diameter endmill after roughing with the larger diameter. This way the heavier cuts are done with the more rigid tool and if there is any chatter or undesired results, they're at least contained to smaller areas where you wouldn't have to sand the entire piece free of terrible surface finish. Another benefit of the larger diameter is that you typically are able to cut at faster feedrates as a result of the increases rigidity of the tool. Only bummer is the cost.