If you don’t need to buy Wi-Fi (which I recommend) because you have it already and you only need a 4G for your mobile phone, I recommend getting it from Orange.
I have a Finnish connection in my iPhone which I turn into Wi-Fi at my AirBnB but keep it on when I’m outside since the prices are the same inside Europe up to 10Gt a month. In the time of WhatssApp, Skype and Facebook calls you don’t need to worry so much about phone bills.
Even so, I wanted to have a Spanish number because of taking care of things and for the work practice. I didn’t have to pay for the card itself but uploaded 20,00 euros to the SIM which by the way works for 12 months. If you are staying longer, get a real connection instead of Prepaid.
My Prepaid has internet, international calls up to 200 minutes and roaming in EU. I have been using it six days now and I still have 10,00 euros left. I only use WhatsApp for messages but I believe Googlemaps has eaten quite a big chuck of the internet, so I think I will only use my Spanish number for WhatsApp and my own for everything else.
I think for me at least this is the cheapest option.
Next time I tell you about Bilbao where I went for a few days before my practice started and then well, I tell you about Fundación Tomillo.