I did the exact same thing with my book. I wrote it in 3rd person, switched the WHOLE THING to first, and then switched it back to third so I could add secondary narrators.
I think it ultimately depends on your character and your story, to be honest. My main character, for instance, is both very self-aware and a wee bit egotistical. Therefore having the story in 1st person was a bit overwhelming to read because the reader was getting pelted by "I" this and "I" that and "As for me." And the story felt a bit spoon fed. With third person I found that I was able to have a bit more control on the filter between the character's mind and the audience. It doesn't feel as much like a soliloquy now. Also, once I switched it back to third person, I realized that the story (it's epic fantasy) was getting bogged down with the other main character just sitting down and telling my character what was going on in other parts of the book. So when something is happening elsewhere, I let the other character narrate it. But this is because my story is a "big picture." It's about more than just one person.
Take the musical "Grease" for instance. Is it all about Sandy and her romance, or is it about the whole school? If the story you want to tell revolves around your main character, then use first person. If it concerns other people, then maybe 3rd person would be better.
Harry Potter is also a great example. It is written in 3rd person, though it's almost all from Harry's point of view. However, if it had been in first person, the reader would have been so bogged down with Harry's internal struggles that we wouldn't even have really noticed what else would have been going on in the book.
The opposite goes for Twilight. It is in 1st person from Bella Swan's point of view. It is ALL about Bella and her relationships with Edward and Jacob. Even though other things are going on, it all only matters because it determines whether or not they get to stay together. If this book was in 3rd person, even from Bella's point of view, the story would automatically open up a bit more and you'd be invested in the story for the Cullens as a whole, instead of just Bella and her romance.
I used those books as examples because they are popular, mind. Not necessarily because I thought they are the best examples of literature, but hopefully you get my point.
If in doubt, pick the most important chapter in your book (probably the climax) and rewrite it into first and see if you like it better. For all you know, you might try it and hate it.