I don't think T-Mobile is an option for Canadians, but it's a good option for Americans. I used T-Mobile a few times in Europe, but in the future I'll buy local SIM cards as I travel, something I've also done a few times, though never in Spain or France. In France I used my T-Mobile which roamed on the Orange network. My default purchase would probably be an Orange SIM card in France, though I'm not sure they are the best. I usually go with the cheapest SIM - not necessarily worried about having amazing cell phone reception on vacation, just something that works most of the time.
You can roam in the EU on another country's cell network for a capped roaming fee, so you could buy a SIM in one country and use it in the other. If you use all the data or minutes and need to re-charge, it could be cheaper at that point to buy another SIM in the new country.