We have never purchased a SIM card in Spain. We were told that it is not the same as buying a SIM card in the UK. Anyone have any recommendations which brand to buy, where in Madrid to buy it and how many GB we would likely need for 21 days. Or should we buy one on amazon before we leave the US. Thanks in advance.
I like this blog for European SIM info per country. Here's the one for Spain, telling you about the different Spanish mobile operators:
http://prepaid-data-sim-card.wikia.com/wiki/Spain
I've never been to Spain (except to Madrid's airport in May to change planes, where I saw Vodafone SIMs on sale at one of the stores inside security; not sure if you can find one as you deplane/leave the terminal). As in many other European countries (not the UK), SIM cards must be registered with a passport. Not a big deal - they surely sell them to tourists all the time, but it may take a few minutes. I suspect you are better off going to a branded mobile store (Orange, Vodafone, etc.) where the clerk speaks some English vs. trying to buy a cheaper one at a smaller store - but if you are fluent in Spanish, it might be doable at one of the smaller stores.
Just make sure your phone is unlocked before you leave the US. Verizon FYI still doesn't lock its phones. AT&T does. If your AT&T phone is paid off, they will unlock it for you by request. This simply means typing in the unlock code one time after you've inserted the new SIM and that's it.
Be sure the SIM card works before you leave the store - make a call, try surfing the web, etc.
Some people use a lot of data, but I guess I don't. I can get by when I travel using about 100-120MB per day, using Google Maps almost constantly as I walk around or use public transit. I usually use WiFI when it's available to save data. And I don't stream video while on the SIM - that eats data quickly. So for three weeks, 3GB would be plenty of data for me. If you are the type of person who uses lots and lots of data, maybe you'd want more.
If renting a car and driving, if you plan to use your phone as a GPS, consider using an "offline" map system like Google Maps "offline" maps to save data - you can download the map ahead of time and keep the phone in airplane mode while driving to save data.
FYI, to make calls home to the US, you can use Google Hangouts (also install Hangouts Dialer if you have an Android). Hangouts lets you call US numbers, even landlines, for free, while on WiFi or using your SIM (uses about 2MB per minute of data). Just add a +1 to the front of US numbers to call them from Spain. You may think you'll never need to call a landline phone, but it's super handy to be able to make long calls (wait on hold?) to your bank or airline in case something comes up - and not worry about draining your SIM's minutes or something.
If you want to buy one before you leave, I recommend the Dutch Vodafone SIM (Dutch SIMs need not be registered with a passport). You can roam with it to Spain. Here's my write-up from last year. I most recently used the SIM in Portugal but also at the Madrid airport while changing planes.
Most of this is still accurate since last year.
https://community.ricksteves.com/travel-forum/tech-tips/dutch-vodafone-sim-card-for-use-in-europe
It's a bit more trouble than just buying one in Spain when you get there, but it is nice to have a working phone the minute you land in Madrid. If you can wait until you've gotten settled after arriving from the airport first then find a mobile store, I'd just wait until you get there to buy one.