Not only are mini split more efficient, but the difference in air flow and capacity are huge. Even though both systems may have 18000 BTU, there is a significant difference in what you'll expect come time to turn it on. Some systems are quite affordable, we offer some starting at $619.
Lets say you do most of the install, and just have an HVAC tech vacuum down the system for $100, you can get away with $750 for the whole system installed. That's extremely affordable compared to some window systems, and you've got significantly more cooling power. Many of our systems also offer low ambient cooling, which means you can still cool your grow room, when the outside temperature drops to 5F, which is important if your setup is in a colder climate.
You can see our 9000BTU system here
http://www.sogoodtobuy.com/9000BTU-Ductless-Air-Conditioner-p/senl9000.htm but most of the time you'll probably need a 1.5 ton or 2 ton system, which is also available for a little more.