The question is what you need the sim for? Data mainly? Data+Voice? How often do you plan to use a regular phone call? With data you can use whatsapp or similiar apps to call friends and family.

Dataroaming in the EU includes all EU countries plus very often UK, Switzerland, Norway, Iceland and Lichtenstein. So with a French or German SIM you are okay for dataroaming in all countries you will visit. In Germany the three large brand are T-Mobile, Vodafone and O2. In addition there a hundreds of virtual providers that sell under their own brand names but use one of the mentioned three main networks. All brands offer prepaid packages which is in your case the most convenient way to go.