I live with a sun conure and I've met a senegal on several occasions. I can only speak from personal experience but the biggest difference I saw was how active they are. My sun is an energizer bunny (though he has mellowed out with age). The senegal I met was more laid back. My sun has a much louder, high pitched, piercing screech but it is shorter in duration and non repetitive. The senegal I met has a lower decibel (but still loud), high pitched screech that is longer in duration and tends to repeat. Funny enough, I can stand my sun's screech more because even though it is louder, he rarely goes off. The senegal I met seemed to go on and on. But again, this may be just that senegal. Others may be less vocal.
Some suns are more open to other flock members, senegals (from what I've read) tend to be more aggressive to other birds in that they would outright attack. I have 2 parakeets who follow my sun conure everywhere. He lets them visit his cage and eat from his chop dish. He goes to visit their cage and eat from their chop dish too. He just doesn't want them preening him or snuggling close to him. He's a bit of a diva.