if...else within JSP or JSTL

The construct for this is:

<c:choose>
   <c:when test="${..}">...</c:when> <!-- if condition -->
   <c:when test="${..}">...</c:when> <!-- else if condition -->
   <c:otherwise>...</c:otherwise>    <!-- else condition -->
</c:choose>

If the condition isn't expensive, I sometimes prefer to simply use two distinct <c:if tags - it makes it easier to read.

Should I use JSTL ?

Yes.

You can use <c:if> and <c:choose> tags to make conditional rendering in jsp using JSTL.

To simulate if , you can use:

<c:if test="condition"></c:if>

To simulate if...else, you can use:

<c:choose>
    <c:when test="${param.enter=='1'}">
        pizza. 
        <br />
    </c:when>    
    <c:otherwise>
        pizzas. 
        <br />
    </c:otherwise>
</c:choose>

In case you want to compare strings, write the following JSTL:

<c:choose>
    <c:when test="${myvar.equals('foo')}">
        ...
    </c:when>
    <c:when test="${myvar.equals('bar')}">
        ...
    </c:when>
    <c:otherwise>
        ...
    </c:otherwise>
</c:choose>

If you just want to output different text, a more concise example would be

${condition ? 'some text when true' : 'some text when false'}

It is way shorter than c:choose.