Ask HN: Dream job emails back shortly after accepting "very good" job. Thoughts?

In October of last year, I applied to a handful of companies, one of them was something of a stretch (Game dev, where my background is systems coding/sysadmin work), but would involve working on a project I'm both professionally and personally crazy about. Over the last 5 months I sent 2-3 queries as to the status of my application, I got through their round 1 within a week of my initial app, but then complete radio silence. There was a time sensitive offer from another company (pick your favorite Google/Facebook/MS style bigCO) which I chose to accept (legitimately an interesting position, and certainly be a positive career move); I signed my paperwork, have begun talking to my team, and accepted my signing bonus.

Within the last few days,(a month after accepting ), I heard back from the "dream job", saying they wanted me to have a formal interview. I had, on accepting the bigCo offer, sent an email announcing this to the other companies I had been pursuing, but apparently this went into the same black hole as the other emails.

This puts me in something of a strange position. I feel in some ways compelled to continue at bigCo; call it my personal programming to finish something I set myself to full stop. That being said, I worry that I may be turning down something I could be passionate about for pragmatism. (And how do I even know, passion or not, that it would be a good fit? after all, 5 months completely incommunicado doesn't bode well.)

There's also the option of taking the interview to "test the water" and be better prepared for future such positions, (and for the one in a million chance that they throw me an offer I can't refuse...), but I worry that I'd wasting their time for what is frankly a long shot (given bigCo).

Far past rambling at this point, thanks for bearing with me, any thoughts would be welcome on how to handle this professionally.