A dictionary says both chengyus being equal in meaining. Is it so, or is there any difference between these two idioms?
Both of them are correct, they share the same meaning.
拔苗助长 is widely-used nowadays.
揠苗 : 1.见"揠苗助长"。
助长 : 1.帮助生长;促使增长。
The correct and commonly use is 拔苗助长 ( bá miáo zhù zhǎng ) here is the mean of that phrase