I live > 250km away from these cities but I can give some suggestions.
For cultural visit, Koblenz has more to see than Frankfurt. It is a city surrounded by mountainous area and a large river. On the mountainous area, you'll find many ancient castles which you can access easily. The view from the top of these castles is stunning.
This region has its own long standing wine culture. You can grab one bottle of wine back to Malaysia
Currently they have a kind of fermenting wine called Federweiss in season, which is only available for a very short periode of time every year. But surely not in October.
Frankfurt is more towards metropolitan and administrative city with skyscrapers and of course many shopping streets. There're some famous restaurants there (which I haven't visited yet). The night view of Frankfurt am Main is nice.
Don't forget to bring enough clothes and a rain jacket with you, the weather can be quite harsh and unpredictable.