I would go with the Mitsubishi if you have to buy a tractor NOW. I agree withe everyone else, that weight and HP are most important, other than the reliability factor. I personally wouldn't buy a Iseki, and would wait for something better to pop up on the market if you're concerned about not having sufficient weight and HP with the Mitsu. I know people with 18hp scuts that do all the work you're talking about, and weigh the same as the Mitsu, and rave about the performance.
If you have to choose now, either tractor you'll get will probably need wheel weights, loaded tires, or a counter balance if you plan on moving full buckets of gravel. A 18hp CUT will run a post hole digger, 4' tiller, single bottom plow, 4' brush hog, 4' box blade, and the list goes on.
Chris