How loud do you like your movies? That's a long room. I sit about 12' from my 170's and the room is 18ft. long x 13ft. wide with 8ft. ceiling. I'm using a 65w/channel outlaw 1050 receiver and have plenty of volume.
I have not auditioned the 340's so I cannot comment on them. I absolutely love my 170's, so if the 340's are better I would never know it unless I directly compared both.
The 340's will play bigger due to their extra driver and size.
You could always try the 170's for the trial period and switch to the 340's if they don't fry your bacon!
Ed
* Sierra-2EX's W/V2 crossover upgrade
* (2) Rythmik F12's
* Parasound Halo P6
* Audio by Van Alstine DVA-M225 Monoblock Amps
* MiniDSP 2x4HD For Sub calibration
*World's Best Cables Canare 4S11 speaker cables