I have used the American Marine unit. I have had one for about 8 years now and it still works well. About $199 US by Mail Order. I'm not familiar with the Aqua-Medic unit, but I would think it would be nice as well.
I keep planted tanks, and personally prefer the Milwaukee Instruments model SMS 122 Controller (got three of them). The AM unit has a high and low set point, and is made for use by reef keepers. The low set point is for activation of kalkwasser type additives that raise pH. The SMS 122 is a high set point only, made for CO2 systems (planted) setups primarily, but it is a LOT less expensive than the AM, about $90-$100 US by Mail Order. The AM unit measures to two decimal places. The MI unit measures to one decimal place, but in a planted tank I don't feel the need for accuracy to the hundredth; to the tenth is fine for me.