I would have to say the main decision maker on these two would be the noise. Many people cannot handle the screams of a sun conure on a daily basis and become annoyed with them, give them less attention etc. Senegals do stay much quieter. They may at times become loud but not usually at the volume of a sun conure and not for as long (I happen to love sun conures but the is just me LOL!)
As far as temperment. Sun conures can be quite gentle although many become nippy with breeding age. Senegals do as well sometimes show breeding nippiness although I haven't noticed it as much in them. If handled correctly, given lots of interaction, a good diet, and a variety of toys either bird should stay tame and make a good companion.