These are made at exactly the same place. The second is heavy: the back of the tongue lifts and the sound fills the mouth.
سSin
The tongue tip and lower teeth
The tip sits just behind the lower front teeth and the air is forced through a narrow channel, producing a whistle. Ṣād, sīn and zāy share this whistle.
صSad
The tongue tip and lower teeth
The tip sits just behind the lower front teeth and the air is forced through a narrow channel, producing a whistle. Ṣād, sīn and zāy share this whistle.
Both come from the same place in the mouth — what changes is how the tongue is held, not where it touches.
Two words, one letter apart
Everything else in these words is identical. If you can hear which is which, you have the letter.
سَيْف
sayf
sword
صَيْف
ṣayf
summer