There are so many models from BMW E90 & Audi A4. You need to be specify. Both models also got their pro & con

BMW E90 320, 323, 325, 330, 320D, 335, M3
Audi A4 1.8T, 2.0T, 2.0T Quattro, S4

From my personal view BMW got more spare part (new & half-cut) support compare to Audi and more reasonable priced in aftermarket nowadays. Both reliability is also soso..

My personal pick
BMW E90 320d LCI model
But will need to standby around 5k for timing chain replacement and other minor fix.

both the audi and bmw have a lot of parts. The trick with the VW group is that the parts are the same, only different branding. So most of the time you will be able to find a VW part that is the same as the part you need. Not many know this but for porsche owners can help them save a bundle.

For BMW you do have a lot of parts, but its the same as VW only BMW doesnt have another brand for their parts. For both brands you can find aftermarket parts as well. the problem with BMW is that they use plastic inside so things like pipes and such, so make sure you inspect and replace before you get a catastrophic failure. Ideally you can replace with OEM metal parts for things like pipes but for cooling systems or similar, i would becareful about using metal pipes since plastic might be used here to avoid reactivity with the chemical based fluids, but also means that you need to be vigilant to replace the plastic every 5 years. Thats why chemical battery cases are plastic otherwise the acid/salt will wreak havoc with the metal.

You can also get parts for BMW from recycling where the car is torn down into parts to be recycled, and its here you will see just so much plastic comes from the engine bay. BMWs are plastic.

Thats why unless you have a gun to your head, dont pick either.

go for the e90 320d. on average, 800km per full tank for city driving wld make u smile from ear to ear. plenty of parts.

rule of thumb, set aside some cash for wear n tear replacement. better if u can source from an owner himself, with full service records.

You will definitely see more BMW on the road than Audi, the E90 sold quite well