As far as I know, there's no appreciable performance advantage to JSTL, but, unlike Struts, it is a standard, so if you use the JSTL tags you're guaranteed to have support for them far into the futue.
I agree with Colin. There isn't an appreciable performance difference between the two tag libraries, but JSTL is a standard part of J2EE, and Struts tags are not. JSTL tags are also much more powerful than Struts tags. I generally use Struts tags only when there is no equivalent JSTL tag (<html:xxx> tags, for example).