I have never played a Squier Affinity bass, but I have plenty of SXes and I have a Squier Deluxe V and I have played the vintage modifieds in the store (they feel like my Deluxe, IMO). Anyways, I agree with theory.
But the OP asked the wrong question. "Best" depends on who is asking as well as what they plan to do. The neck on SX basses is chunky, some people like them, some don't (I obviously do). THAT IMO is a bigger deal than the other differences people often bring up on value basses.
Even though I have never played an Affinity bass, I am pretty sure their necks are not as thick as SX necks. The most important thing that I can NOT comment on is the Affinity pups. But on the SX pups I will only say this, I have a few SX basses (see sig) and I have replaced pups on 3 of them. But I have NO intention of changing the pups on the others. The stock pups are strong and I like them!
But I WILL say this, if you buy an SX, set it up (or have it set up, which might have to include leveling frets) and change the strings you will have a good bass. AND you will be happy as long as you like chunky necks!