You could get one 500 for less than 2 350's. Two subs can help you get a better blend of LFE in your room. You can put them on opposite walls/corners. I have a very large room as well, 900sq ft with 9ft ceilings. When I owned a 350 it did well, but the 500/600 are in a class by themselves.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85