Cell Phones in Spain - Any brands recommended - Better to purchase in US or Spain?

Jc,

If you have an unlocked cell phone already, you can take that on your trip & just purchase a SIM card there.
If you don't want to take your smartphone along, you may have an old cell phone that you can take with you.
I purchased a Lebara SIM card at the Atocha train station (in 2015) for my old unlocked flip phone.
I received 400 minutes of call time (domestic & International calls) for €10/month!

I purchased my SIM card at the 'Telecor' shop in the Atocha train station in Madrid.

Here is a map of the Atocha station. The 'Telecor' shop is labelled #16 (blue), on the ground floor (Planta 0):

Adding to Priscilla's excellent advice, what do you want the phone for? Asking for a cell phone brand is like asking for a car brand - we need a lot more information to give good advice.

If your phone doesn't work in Europe, will you be wanting one that just makes calls, or does it need to be a smartphone? Will this be your only trip to Europe, or will you want a phone that will work in other countries? Do you want to get a high end phone you could also use when you get home (so you don't mind a high cost), or just one for travel (if you're happy with your current US phone)?

Most of the same brands are sold in Europe as in the US, but the models can differ. If you want a basic phone (calls only), it's better to buy it in Spain, so you know it will work there (less expensive phones are not always "world phones"). If you want a fancier phone, you can buy it here or there. Buying a phone here has the advantage of being able to program in your numbers in advance, a great time saver.

Or were you asking not about the phone (handset), but about the carrier? Lebara is one carrier with low costs, but there are other companies. Again, depending on what you want to do with the phone, that will determine the best deal.

Who is your current US carrier, and what is your current phone model? And how do you plan to use the phone in Spain? Depending on these answers, you may do fine with just getting an international plan for your current phone.

Would also try and bring an unlocked phone.
Good thing about Lebara is calls between Lebara and Lebara are free.
They also have inexpensive data plans.
You can add money online at their website.

As others have noted, make sure your phone is unl;ocked. If you don't want to use your at home, everyday phone, you can get decent android, unlocked phones reasonably inexpensively.

You can either buy the sim card here before you go, or ijust about everywhere when you get there. IGetting it there might be preferable as you can have them install it, and they'll make sure it works. I just returned from almost a month in Spain, and had good coverage everywhere with a card from Orange, I bought it as soon as we got out of the airport in Barcelona, and had it through all of Northern Spain, Extremadura, Andalucia, and Madrid.