IMHO, Traveller's Checks have gone the way of the dinosaur. Sure, you can get them, but they're a pain to cash out if you're planning to get cash for them and not use as purchases. Some places actually charge fees to cash them, and if you bring US$ ones, be prepared for some very non-favorable rates of exchange.
ATM cards are good for cash. ATMs are much more readily available, and many are open when the bank is not. I've had positive experiences everywhere with ATMs, and I can say for certain that in Cadiz and Barcelona, they're all over the place. Check with your bank to see what they charge, so you're not in for some sticker shock.
Credit Cards might be an even better bet for purchases overall. With the exception of some bars I went to, credit card use was fairly ubiquitous. Some machines you may encounter may require chip-and-pin enabled cards, but if there's a human around, you'll likely have no issue. I have one Capital One card I only use overseas because there's no POS fee or additional charge for currency conversion, and their rates are very close to interbank. Everywhere I wanted to use my credit card, I did, with the exception of buying a train ticket, which required a chip-and-pin card. The office was open, so I just queued and swiped my card with the clerk.
Wells Fargo has a decent program for purchasing cash, and the rates are generally OK (it's about a 5% markup between interbank rates and what they're selling for) if you're going to buy more than $1000 worth - for free shipping. Many banks will charge you 1-3% on top of the interbank rates for ATM use, so keep that in mind when weighing your options...
https://www.wellsfargo.com/foreignexchange
Here's a chart to do some preliminary research for US/Canadian cards:
http://www.flyerguide.com/wiki/index.php/Credit/Debit/ATM_Cards_and_Foreign_Exchange
-Steve