Either. Either way is correct. Your first version is perhaps a bit more British, but both are used on both sides of the "pond."

Your choice, but "ee" is more common. The other often sounds a bit pretentious, at least to me.

Thank you everyone for your input. Peachey you're appealing to the music nerd inside me, thanks a lot for the (late) discovery. Su.Ki you are totally right regarding US media. I started learning English quite young playing videogames that weren't translated, trying to understand lyrics in music or later getting interested in only original versions of tv shows or movies. I've heard both pronounciations within the same tv show so I was curious about it ;)

Can I just to add to what the two American members have said? In standard British English, the first syllable is pronounced in the same way as the pronoun 'I'. The less standard 'ee' pronunciation is also heard in the UK, however. This is becoming more common, due to the influence of US media, especially amongst younger people.

This song might give a bit of perspective: http://www.youtube.com/watch?v=LOILZ_D3aRg