I tried the portable route. Not a good idea. I finally ended up getting a "Mini-Split" AC system. It's great. I still have to dehumidify in the summer, as it does not take out all of the required moisture, My portable may have been under sized, but it hardly had a chance with the added heat from the dehumidifier. The Mini split will keep it where I want it even with the dehumidifier running full. My shop is almost 30 x 10 as well, and is insulated, but not sealed very well, however, I'm now able to control things both ways. The mini split I bought also has a heating capability, and, though limited, makes the temp all over the shop much more even than just using one of the radiator type heaters. Be prepared to spend a couple of bills, though, on the system.
The problem with portables is that they use inside air on the back side of the unit, and exhaust it from the shop through a big ol' 5" hose. That makes the shop, negative pressure, so it pulls in outside air(@90% RH) to replace it. The front side cools just fine, it just can't keep up.
By the way, if you want to try the portable, I know where you can get a 7000 BTU version at a steal. Only one summer of use.
