For Spain there are basicly two interesting prepaid data offers:
- Yoigo.es, who charge € 1.20/day for unlimited access.
Yoigo is a UMTS-only network providing 384 KBit/s with national roaming on Telefonica's Movistar-GSM and UMTS network (they had an EDGE-only roaming on Vodafone until June). SIMs are sold e.g. at the Phone House shops.
- Simyo.es, who are based on the Orange network (supports HSDPA with 3.6 MBit/s) and have two different tariff plans:
- Tarifa Diaria Internet
You pay € 0,09/MB.
If you reach 11MB/day the billing will stop until you reach 100MB/day.
If you exceed 100MB/day the exceeding volume will be billed again with € 0,09/min.
But then again there's a cap of €5/month if you don't exceed 500MB.
As soon as you exceed 500MB you will be billed €5 for them + € 0,09/MB
- Tarifa Plana Internet 5GB
You get 5GB per month for € 24.99. After exceeding 5GB speed is limited to 128 KBit/s. The small print says this tariff is postpaid, however the page also states it's without contract. So I'm not sure, what the exact conditions are and if you can get this tariff as a foreigner.
But I don't know if simyo SIMs are sold anywhere else except for online.
Your Huawei USB modem will work with any Spanish network, as they use GSM900/1800 and UMTS2100, like everywhere in Europe.
UMTS900, which is supported by very few devices, hasn't been rolled out in a noteworthy scale anywhere in Europe.
Btw it would be helpful for everybody, if you changed the topic of this thread into something reasonable like: "recommendations for data in Spain & Portugal".