Today, dollar for dollar, is the 318 faster than the 340 ???

No doubt about it, the 318 can be made to go fast economically. If it's a "one off" go ahead and have fun with it just realize you are probably going to lose most of your investment.If you plan on staying in the hobby to realize future projects you will come out money ahead with the 340 and stay in step as the hobby costs rise.
An old fellow who's opinion i value always said "you have to build/buy what everyone wants!"
He also said "you can dress a 10 in rags...she's still a 10"