The only real difference is in the time it needs to compile.
Scriptlets are added to the generated
Java source verbatim, while tags need to be parsed to generate the method calls (and potentially object instantiations) they represent.
At runtime the difference is probably trivial.
The only real difference is that using scriptlets you're able to get more code into your JSP as tags will typically result in more and longer method calls.
But if your JSP is getting so large that switching from scriptlet code to tags brings you over the maximum size for methods the JVM will accept (64KB compiled code per method) you have more serious problems and should really split up the JSP into several distincs modules using <jsp:include> tags.