Bullet vs Rapid Ratings

That is up to the player, but if that player is somewhat serious about improving at chess long-term: then it is usually better to stay away from bullet. At the very least, until they really build up their pattern recognition and chess ability for longer time controls. I recommend games at least 10 min. and maybe even 20 or 30 min if your schedule allows it. 

Under these conditions, you'll have a little more time to think, calculate, come up with better moves and so on.

The best speed chess players have a massive advantage over a newer player because they can move super fast and accurately in their "autopilot" because they know so many patterns, have opening theory usually memorized and are great at converting theoretical advantages efficiently such as Queen + King vs King checkmate. It is up to you, but I highly recommend working on chess.com rapid rating before embarking on blitz or bullet if possible. A blitz or bullet game here or there might be okay, but one thing you want to avoid is making that your default and conditioning yourself to move very fast - this is a weakness if you can't adapt to longer time controls and slow down effectively.