FYI, hadn't noticed it until now but the difference isn't so much in the power as it's in the transmission and rear axle. The 340 comes with an Allison 2500MH 6 speed, the 350 comes with an Allison 3000MH 6-speed. This is fairly significant because the 2500 significantly reduces your GVWR and GCWR over the 3000. The 2500 is rated to 27910/33000lbs, the 3000 is rated to 29410/39410lbs (i.e. the 2500 can tow 5000lbs, the 3000 can tow 10000lbs). I haven't seen any UCC / CCC numbers published but I suspect that the 350 does not weigh 1500lbs more than the 340 so the 350 likely gives you a higher CCC as well.
See this link for the specs on both
http://www.freightlinerchassis.com/p...R0601-2009.pdf
Whether this is worth $10k or not is debatable (seems pretty expensive to me) - although if you want to tow a large vehicle / trailer than it's a non-issue ...