Aflatoxin is good for reducing your opponent's damage if you lock up their board early, though you probably want to run a variant with three shields (extra such as jade or mirror) to make sure you aren't waiting as long to use afla, unless you want to risk killing yourself. It also depends on other factors in your variant; for example if you include a purple nymph, you may not need aflatoxin as much.
You'd really be fine with just not using aflatoxin or miracle and having one less card in the deck.
Fractal tends to give you more control over what you target, and you can always fractal your own destroyers or an opponent's creature. With mindgate, you can only pick from your opponent's deck, and you can't choose which creature because you don't have eternity. Its cost also builds up quickly, though you can get more cards in the long run. Fractal is just a simple one-time-use source of extra boom.
Fractal is more reliable; mindgate has more potential. Try both and choose for yourself.