Jetty is bundled with Solr due to it's size:
The Solr example app has Jetty in it just because at the time we set
it up, Jetty was the simplest/smallest servlet container we found that
could be run easily in a cross platform way (ie: "java -jar
start.jar"). That does not imply that Solr runs better under Jetty, or
that Jetty is only good enough for demos -- it's just that Jetty made
our demo setup easier.
Users should decide for themselves which Servlet Container they
consider the easiest/best for their use cases based on their
needs/experience. For high traffic scenarios, investing time for
tuning the servlet container can often make a big difference.
I have been running Solr with Jetty for few small sites for months and had no problems.
My vote goes for Jetty.