I'm on the verge of both. After reading and thinking, I think I will go with CO2 tnak outside. Easier to change out, easier to read. The one thing I wouldn't like is that if you had the tank inside, the tank pressure will read lower.
The only thing to worry about is heat. Excessive heat will make the gas expand and possible blow. But we are talking a LOT OF HEAT, 200-300 degree kind of heat. Nothing ambient wise to worry about, global warming included (if you believe in that sort of thing).