Both are under Pos Malaysia but different thing.

Pos laju = Required signature by receiver. U need to pack your parcel and ask for poslaju form from the counter.  Poslaju can post all big, small, heavy or light parcels.

Pos Express = Do not require signature from receiver. Pos man will just drop the pos express envelop in the Pos box. If cannot fit in, posman might just throw it into the house or ring the bell ask u to take it. Pos express only able to post things that can fit in their envelop.
Pos express comes with these envelop:

Both arriving durations is almost the same. but Poslaju is safer than pos express. In case of parcel lost, Poslaju is still able to get back the item but for pos express, it will be hard to get the parcel back.

u cannot use pos express for non-doc items.
plz use poslaju to avoid headache.