I fly my mCPXes in tighter areas... the 300X just seems like it wouldn't be that much smaller than the 450, to justify the cost. Honestly, I *MIGHT* go for the 130X as it's flybarless, brushless, and the tail is driven off the main motor by a torque tube.. it'd also be small enough for most back yards. The 300 just seems like it'd be the worst of both worlds; though not using the crappy linear actuator servos with their loose mounting points would be a big plus.
Then again, my next size-step is likely going to be building a 700.
Really though, for most flying with a back yard to stand in or residential street to take off from, I just go up a hundred feet or so with the 450. Plenty of room in the sky to buzz around, when all you really *need* is line of sight, assuming something doesn't go wrong.
Blade: mCX2, mSR, mSRX, Nano-CPX, 2x mCPX (v1), mCPX-BL, 120SR, 450 X
Spektrum DX8, DX6i