Decisions, decisions

Okay people, I have a dilemma on my hands. I have to make a decision. Not something I’m good at in the slightest…
You see, today I was offered a proper job at the place I work. The contract would be for one year, following which I would be offered an ope-ended contratc, provided I still want to stay. All sounds very good, yes. Then I arrived home to find an email from the company I applied for a position of trainee translator at before Christmas. Guess what? They want me to come for an interview. So, what do I do? I have decided I will at least go to the interview at the new place, but that means taking a day off work. Do I tell the boss where I’m going? Even though he’s made me a definite job offer? And if I’m then offered the other job what do I do, where do I go? Aargh… it’s making my head hurt already and I haven’t even had the interview yet!
It’s a good kind of dilemma though. Really I shouldn’t be complaining. Still a dilemma though, so if anyone has any advice please feel free to give me it…