Hi Nick, If installed and properly maintained, an air handler in the attic should not cause condensation issues. Between the mini-split and full AC system, it is going to come down whichever is going to make you most comfortable and which is best for your budget. Mini-split systems can be more efficient and don't require as much maintenance. We typically recommend mini-split systems for a garage because they are typically more cost effective among other reasons. If you have a contractor offering a full system for less than a mini split, I would recommend researching the company, the product they are offering, and what is included in the price a bit more. It is not often that a full system can be installed cheaper than a mini-split.

I'm assuming that they are recommending 1.5 to 2 tons because of the lack of insulation and sealing in the garage? If you can insulate the walls and ceilings and have insulated garage doors, you shouldn't need more that 1 ton. Minisplits go down this low, while traditional ducted systems only go down to 1.5 tons. If you need that larger cooling capacity, I'd go with the standard ducted system at a lower cost (most companies installing minisplits are making way too much profit off these). If the garage is well insulated and sealed, I'd probably go with the smaller minisplit.

If you only have 1 supply vent and 1 return vent with the full system, there is really not much different between the full system and the mini split. The difference is one has the blower/coil in the attic right above the ceiling, the other one has the blower/coil inside the garage near the ceiling. 2 things need to be considered here. 1. Price: as mentioned, mini split should be cheaper, better shop around first.... 2. Noise level: Full system blower right above the ceiling can make lots of noise. Mini split's blower, even in the room, is much quieter.