The Pixel 3a phones run on Qualcomm's Snapdragon 670 chips, which are said to have comparable performance to the chip running flagship phones from 2017 — the Snapdragon 835.
Google's Pixel 3 phones run on the Snapdragon 845 that powered flagship devices in 2018, which is undoubtedly more powerful than Snapdragon 670 in the Pixel 3a phones.
Still, the Snapdragon 670 is no slouch and should offer solid performance.
Both Google's Pixel 3a and Pixel 3 phones come with 4GB of RAM.
As for storage, the Pixel 3a phones only come in a 64 GB option. You get more choice with Google's Pixel 3 phones, ranging from 64GB to 128 GB.