How humid is the growing environment, Discrete? The downside of portables, whether a/c or dehumidifier, is how often you have to empty the water tank. I ran a dehumidifier during one of my grows and had to empty the water every couple of hours. The unit switches off when the water tank is full, meaning it needs constant supervision. This was in tropical humidity; it may be better depending on your climate.
Also, assuming your tent is not airtight, what size is the room? You'll have to take this into account when deciding how many BTUs you need. For example, for a 20' x 20' room (400 sq ft), you need an a/c with around 10,000 BTUs, maybe $300 - $400 for the unit. They're expensive to run as well: 10,000 BTU = 3,000W equivalent.