When it comes to writing ANYTHING, ANY stylistic choice should be done because it is the best execution for the intention you're trying to convey. If you write in 1st person, it should be because the content of the story can best be conveyed through that tense; if it's 3rd person, then do that.
1st person will give you the dissociate objective viewpoint that will allow you to delve further into a character's mind. 3rd person is limited in this because a person doesn't go about analyzing their own thoughts as they're having them, but the 1st person allows the narrator the omniscient view necessary to describe the action and the subjective view of the character without confusing the reader or breaking the narrative construct. HOWEVER, the 1st person also diminishes the intensity of the subjective experience of a character.
So if you're tying to place more emphasis on ambiance and detail, then 1st person works well. If you're trying to convey the subjective experiences of a character, 3rd person works better. But be aware that 3rd person will also make it easier for the reader to DISENGAGE if the gender or name of the character becomes a significant aggregate in the prose: EX: "Oh man, oh man, this is so fucking hot, it's like I'm right there with...'Margaret'? My name's not Margaret! Ah DAMMIT, now my headspace is all shot to hell!"
You can also use tense to deceptive effect without alienating the reader, but this is an advanced tactic that requires skill and the right type of story to pull off.
Mixing tenses is a stylistic no-no, but I suppose it can be done. One tactic is to write the 1st person in regular type, and the 3rd person in italic type, provided you use paragraph breaks between the tenses when switching so the reader recognizes that there's a break.