If I had to choose one - Italica (Sevilla) or Medina al-Zahra (Cordoba)

It is a coin flip. Choose one and save the other for the next trip. The Medina is clearly unique but it can be seen in a morning or afternoon. Might give a slight edge to the Medina given its position in Moorish history and Spain.

Thanks Frank.
Problem is I didn't time Cordoba well. I had initially planned to arrive late evening on a Sunday from Seville and depart early on Tuesday for Toledo. This was planned before I knew about the Medina. So add this and all the Monday closures and early Sunday closures, I would have to cut either from Toledo, or Seville. Also, as I am not driving, the 'usual' method of taking the tour bus from Cordoba to Medina al-Zahra seems to only be once a day in November which is an 11:00 departure not getting me back to Cordoba until after 3. You can see that the Medina isn't really a morning or afternoon visit....unless I do by taxi. And it wouldn't get me back until after the Alcazar closes for Sunday. I could squeeze the Alcazar (garden is the draw here) in before the Tuesday train, but I am reluctant to commit to leaving Seville 8 hours earlier than planned.

My other concern is that the websites for ordering the Medina al-Zahra tour bus tickets seems out of date (2015) and I can't buy them on the bus.

Well Keith, that was very informative, and may be the reality hit that I need. Thank you.
Pompeii was first 40 years ago and then last time was 2002...so it's been a while. Same with Rome, and Ostia Antica wasn't on the radar at that time. I did see the Mosaic Museum in Istanbul a couple of years ago, but there hasn't been a lot of Roman in my roamin' lately. I've been more consistently south of the equator.
I have 3 weeks in Spain, so not all locations are rushed, but the Cordoba bit and having to book and commit to some train schedules have me fussed.
Here's the thing, I leave Sevilla Sunday morning and can't arrive much before 9:45 (under duress, I could arrive at 7:45). I then have to locker my suitcase and get to either the Red Cross Hospital stop to pick up the 10:55 tour bus to the Medina (that is hoping I can buy on line), or to the Tourist Bureau, buy the ticket and get on the same bus.....otherwise, I locker my bag and catch a taxi from the train station and ask reception to call me a taxi to return me to Cordoba.

YouTube does a good job giving the perspective of the ruiney ruins, btw....

It sounds like you don't have a lot of time, so don't go to either. There are many other good sights in both cities without spending the time and trouble to go to places that are out of town.

I found it very time-consuming to get to Italica by public bus. I've seen lots of ancient Roman ruins in Italy and Israel so maybe I'm a little jaded, but it wasn't worth the schlep. There are some good mosaics and an amphitheater that's in pretty good condition (for a ruin). I went to Medinat al-Zahara with a guide - at that time, about 4 years ago - the TI offered bus service with or without a guided tour. I very much enjoyed the sight but I thought at the time that without the guide, I would not have understood it well and would not have gotten much out of it.

Keith, yes, I finally found the stop near the Rd Cross Hospital on Google maps Street View, and due to the proximity of the station to the stop, I would not go all the way into town.
Chani, thank you for the additional perspective. I haven't had enough Roman ruins in Italy, and I am not sure of an equivalent to the Medina in my other cities.

I guess I'll just decide when in Seville. By then I will also be able to check weather forecast. The Medina might lose appeal in the rain.