Speed: 4g is faster than 3g is faster than 2g
There's no single answer to how long one will last. Depends on usage and coverage. But you are more likely to have strong 2g coverage than 4g, so if you know you don't need the 4g connection you may save power by turning it off (but it depends on your network and location). Also early 4g phones found 4g used a lot of power, but they are improving, so it may also depend on what phone you have. Try things out and see what works best for your.