Should a composite door be wood or aluminium inside? Any thoughts?

Depends.
Could be steel.

Ok. I’ll re-phrase it Which type of composite door is best?

Depends.
Is it a fire door, a security door, a ballistics door, in a school, an office building or a private residential home?

Normal composite front doors are fiberglass moulded front and rear panels which are layered onto afew layers of ply then have a ali plate fitted around the external edge.

£400 should get you a good one

£400 will get you a bottom to mid range they are well above that especially the fire rated 1’s

Further details. The door I am looking for my front door, i.e. residential.

Could have a proper composite door, aluminum external (RAL colour of your choice), pvc thermal break and proper hardwood (American Ash) on the inside. Pricey though.

you just want a standard composite door then unless you live in a high crime area or fancy paying more for a fire rated.

Even with the added security in the panel and all the locks in the world a criminal can still smash your window…