Jim - I've waited a few hours to see if someone has posted a response to your message, and not seeing one I figured I'd be the first to chime in.
While I don't really know you, now have I interacted much (if at all with you) there is a passion which draws us close and we spend many hours a week doing very similar things and on Reef Central, so I feel at liberty to give you my opinion and recommendation as a 'virtual friend', basing my opinion in my own experience and feelings.
I grew up traveling (went to 6 schools before 3 different countries before graduating from high school) and moved from place to place quite a bit. During college I also traveled quite a bit and had the misfortune of experiencing a 'long distance relationship' and its demise.
I am now much more settled in the US and very happily married for almost 2 years, and I have my new 180 gallon tank! I still travel on business, but its a fraction of the travel I used to do in years past.
Your profile does not state your age, but I'm 31 and I assume I'm talking to someone around my age. My most sincere recommendation and advice would be to listen to your heart and not to let go, or even open the door to tension and conflict which can ultimately lead to the end of a very special relationship.
If the woman you are with is truly 'the one' and the love of your life, and you are committed about your future together, I would pick a few years in Japan over keeping my dream tank intact.
You will always be able to come back into the hobby and build or buy something bigger, better, nicer, fancier, etc...
In love, it doesn't really work that way unless you are fond of "mail ordering"
LOL all jokes aside, I don't think you can pick and choose in a relationship like you can something material, tangible and with a dollar value to it. Sure there is emotional value and attachment and effort that went into it, but it can be recreated... at least more easily than something between 2 people. Love.
I had the fortune to spend a week in Tokyo in 2006. Its an amazing place. Its very different (I don't know how much traveling you have done and how much you like to do so), but it will be very rewarding and culturally enriching.
I say, go to Japan, have a blast with your soon to be wife, experience new things, and all the while plan your next dream tank for your return.
Besides, imagine how many brownie points you'll be making by going! You should be able to double your water volume, have a huge fishroom and the works!
I'm sure its very hard to let go, and break down something you have build with your sweat, blood and tears, but your hobby is yours and you can put it on pause. I wouldn't dare try that with love! I wouldn't want to risk losing something that special.
I hope this has been helpful and not just a long babbling mess.
Best of luck making your choice!