If you ever plan to run e85 or want over 700 hp or even mid 600's on pump gas and you're buying new, might as well get the 2.9 imo. 2.3 is a very capable blower but if I had to buy both new I would pony up $400 more for the 2.9. Resale value would be better down the road if you decided to change the setup anyway.
Id like to buy as many used parts as possible, but what ever is recommended to buy new, I will.