Better? That depends what you mean by "better"...
Assuming the same dielectric (X7R)...
The 0603 cap may be better because it's smaller, so it'll fit where you want to, and it will also have lower inductance, also probably cheaper.
The 1812 cap may be better because it's larger, so it'll have more dielectric volume, which could imply a lower loss of capacitance with applied voltage. It could also have lower ESR, which may or may not be "better". However this is handwaving, fortunately some manufacturers give accurate specifications (in this case you have to click the "C-DC Bias" button, it will also show ESR and everything else).
A 1206 cap could be "better" because it's easier to hand solder than a 0402 cap.
If you need high voltage, a longer cap will offer more creepage distance between electrodes.
If there's a lot of vibration, a larger cap may be more prone to cracking.
Also, a component that is in stock is usually "better" than one you can't have.