Nash is just an overall powerful character whether turtling, rushdown, or a mix of both so you can’t go wrong with him. Probably a better online character when you don’t get the chance to get reads on people since you just play them once and then they’re gone.
Cammy is a bolder character that can be played with basic footsies but is at her best when she picks spots to dash in and acts as the aggressor. She’s also one of the characters privileged with a true invincible reversal which is hugely powerful in this game, meterless at that.
They also both share the fact that they have excellent supers that are easily landed whether in combos or done raw.
Since you say you’re a beginner I would go with Cammy rather than Nash. Not because Cammy is better for beginners, you would win more with Nash early on, but because she’ll take you out of your comfort zone and force you into situations that make you learn. You’ll have to learn good footsies and learn how to push people to the corner. With Nash you’ll probably end up picking up bad habits and just throwing projectiles when you aren’t sure what to do.