Jet Tip:
Never underestimate the heatseekers, people still fall for simple tricks. Get lock on, change weapon to cancel lock on briefly then get right back on the heatseekers and people will often flare the second the solid note stops and you haven't even fired a missile.
The main reason heatseekers are good though is when used with a wingman or even chopper. One of you can force the enemy to flare/ecm and the second just swoops in for the kill. This is much better then the 2 of you using cannons as you don't need to be accurate so player skill is a none issue and the enemy has less time to take evasive. Similarly in a dog fight if your struggling to get enough cannon hits you can swap and force a flare so your buddy can clean up.
You'll need your cannon if you're flying solo without a doubt, but the combination of 2 jets with heatseekers is often much more reliable. As per usual in Battlefield, teamwork is more powerful then any 1 man set up. That said if you catch a guy off guard who is flying quite smoothly you'll wipe them out with cannons once you have the accuracy. The better you get the more you can use your cannon, use the heatseekers as an extra way to put the pressure on in a dogfight.