I would suggest option 1. While the template/dsp/jsp facilities are okay, you’ll be better off scalability-wise keeping the UI in Tomcat and connecting to IS. I would further suggest not using the IS API to do the connection and instead use HTTP posts of XML docs from Tomcat to IS. This will allow load-balancing, fail-over, etc.
If performance becomes an issue, you may need to use the IS API.
Out of curiosity, why are you replacing something that already is in place and works?