I think the better overall investment if you are looking at amount of total dollars Matsu will make from today until the day he retires is to hit the market as a FA. Lets assume for a moment that Matsuzaka signs with the red sox at the age of 26 for 6yrs at 10 mil per. That is 60 mil total until he is 32. When he is 32, he will likely not get a big contract because he will be considered Pedro-esque in size and stuff. That means he may get a Pedro deal of 4 yrs 50-60 mil, lets call it 60 mil. At 36, he will likely be done. So, to recap, you have a guy who has the next 10 yrs of his career planned out at 110-120 mil total, assuming he has a solid career and earns that second contract.
Now assume for a second he goes back to Japan and barring injury is the exacts same pitcher as he was this past season. He will likely get 10 mil for the next 2 yrs in Japan, then he would hit the states as a 28 yr old Japanese Pedro. He would get a 7-10 yr contract for 15-20 mil per to a team he would have a choice to go to. That is 105-200mil for doing nothing in the states, hence he will not have to earn that second contract. THAT is the big draw. That is the big money and that is a gamble he should be ready to make if the sox lowball him.