Using Madrid as hub?
 

Would it be possible to use Madrid as a hub and take daytrips to Cordoba or Seville or others by train? Would May be too hot to travel?

Weather in May will be fine and shouldn't be too hot.
I would not use Madrid as a hub for going to Seville, as it takes 6 hours by train round-trip, and you won't do the city justice with a one-day trip. Córdoba is on the way to Seville.

What you could do is have a full day in Córdoba (AM train Madrid>Córdoba, PM train Córdoba>Seville) and then stay a few nights in Seville.

I wonder what I would do with the luggage?

great question...say I fly into either Madrid or Barcelona.

Is either city good to use a base for day trips.....as this will be my 1st time to either city.

Any suggestions of where to stay say...that are a 2.5 or 3.0 star hotel? Is Ramblas a good place to visit too?

Do you mean when doing a day stop in Cordoba?

There are no luggage lockers inside the station, but there is a place accross the street with lockers that I used when I was in Cordoba last time.
http://www.cordoba24.info/english/ht...roviarias.html

If it will be your first time to either Madrid or Barcelona, why not get to them first, and save your day trips for another time you visit Spain? There is just too much to do in both places to miss out.
In Bcn, a decent place is Hotel Vueling. I was there 2 weeks ago and enjoyed the time. Good location, just off Ramblas, great breakfast, and cheap.

We just returned from a week in Madrid, and did three day trips by train. Toledo, Alcala de Henares, and Aranjuez.

All were easy day trips from Madrid. There were nearly no tourists in Aranjuez, and we had the best meal of the trip in the old market.

I would say that using Madrid as a base city makes some sense, particularly for Toledo and Cordoba, if one can accept the abbreviated visit to the other cities and the associated travel time.

Cordoba on the fastest train is 1 hour, 40 minutes away. Seville takes 2 hours, 20 minutes on the fastest train. Toledo is closer. The train trip itself is worthwhile, demonstrating fast train technology for those who may have an interest, along with seeing the countryside.

I'm planning to do a day trip to Cordoba from Madrid later this year to see the Mezquita, which eluded me on a previous trip to Spain, rather than moving everything down there and staying in Cordoba.

I would recommend the Hotel Europa in central Madrid as a medium-priced and centrally-located hotel right on the Plaza del Sol. There can be some street noise, but it was acceptable.

Based on this information, would it be correct to assume a train is a lot more efficient than trying to fly on the MAD-SVQ route?

Depends if you are already at the airport or not.

From the city, definitely better by train.