Back in November we spent 3 nights in Selçuk after visiting İstanbul. Then we returned to İstanbul and spent a couple of nights in Kilyos before flying back to the U.S. Both Selçuk and Kilyos were very enjoyable! If you're spending 1 night in İzmir simply because you're flying from there the following day then here's an alternative you may wish to consider:
Stay in Selçuk rather than İzmir - then take the train from there directly to the İzmir airport (takes about an hour). To travel from Kuşadası (where I think the RS Tour ends) to Selçuk cheaply go to the main bus station and find one of the mini-buses labeled Kuşadası - Selçuk (it's what the locals use rather than the expensive taxis). You may also find these mini-buses at Ephesus that will take you to the House of Mary (close by) and/or Selçuk to see the Ephesus Museum as mentioned in the RS Tour description. Both of these sites are well worth visiting as is the Basilica of Saint John in Selçuk. What's left of the ruins of the Temple of Artemis (one of the Seven Wonders of the Ancient World) is also in Selçuk (not at Ephesus as the RS Tour description indicates) but you can also see it in the distance from the Basilica.
If you decide to stay in Selçuk, and it's a fun little town to explore, then I suggest is visiting the Ephesus archeological site and House of Mary as part of the RS Tour and then go back and relax/explore in Kuşadası. Then the next morning go to Selçuk, visit the sites, and explore the town. You may also have time to visit the fun wine/tourist town of Şirince in the afternoon (look for the Selçuk - Şirince mini-bus at the bus station). It's up in the mountains just a few miles to the east.
As part of our trip we did consider staying in İzmir and visiting ancient Smyrna and other historic sites; however, you can't do it all. If you do stay in İzmir rather than Selçuk I'm sure it will be a rewarding experience.