Mini split sizing for variable size room

I was considering getting a Pioneer Ductless Wall Mount Mini Split INVERTER Air Conditioner with Heat Pump, 18000 BTU (1 Ton), 15 SEER, 208~230 VAC for cooling a room. The room is about 14x14x10. This room is connected to another slightly smaller room which needs cooling only when we have guests.

Ideally I need the one unit to cool the one room all the time and both rooms sometimes. I am told that the inverter function will allow the unit to scale itself to keep the proper temperature regardless of which size room it is being used in.

I am trying to decide between the 18000 and the 12000 btu unit. I currently have a portable unit by delonghi that is the biggest piece of garbage ever to come out of china, it leaks everywhere despite being "drain-free" and it has no input tube so it just sucks hot humid air in through all the cracks due to negative pressure... Anyway... that thing is around 12000 btu I believe, and it is able to cool the one room very well, in fact it cycles frequently, leading me to believe that it could be smaller and still do the job just fine. Honestly I think I could get away with a 5k window unit for the one room.

The mitsubishi sizing info page says that 12000btu should be sufficient for up to 500 sq ft, whereas my rooms would only be ~200 or ~400 sq ft

I don't mind paying the extra few hundred for the 18k mini split, but only if the inverter will allow it to scale properly down to cool the one room when I only need the one room cooled.




Thoughts?