In my opinion, JSTL along with EL is more powerful than struts tags.
for ex,
struts tags bean:equal, bean:notEqual, bean:greaterThan, bean:lessThan, bean
resent, bean:greaterEqual, bean:lessEqual can be replaced by single core:if JSTL tag along with EL.
So It is preferable to use JSTL EL whenever there is a functional equivalent JSTL EL available for Struts tags.