What is the weather like in southern spain during july-august?

what about northern Spain?

which would be more enjoyable?

which beaches are better?
The weather is very hot in all coast cities of the Mediterranean and South Atlantic. Sevilla and Cordoba can reach 40 and so degrees (104 farenheit). All South and Levant are very hot in Summer. Nights are hot too, altough a little bit less, of course.
Yes, North Spain is colder. In Summer is warm (not as hot as South Spain), but usually changeable, and nights are considerably fresh.
North-Spain coast is a little unknown for foreign tourist, but has a stunning green landscape, full of forest and mountains, a coast with rocky cliffs and large sandy beaches. Beautiful and fisher towns, The way of St. James, sanctuaries, excellent local cuisine, and obviously, nightlife (That's Spain!!!)
Mediterranean and South Atlantic coast are more mass tourism oriented, and notably hotter (as I've said). There are beautiful towns and cities too, good cuisine and especially a vibrant nightlife!!!...
It is as good as the North, but the North is less known and less spoilt by tourism.
Every part has its enchant.

I can recommend you the provinces of Murcia and Almeria. Are the mediterranean provinces that are less spoilt by tourism, so they haven't lose the Spanish enchant. There are also beautiful cities, good nightlife and food, landscapes (in Almeria, the only dessert in Europe), urban beaches and wild beaches, natural parks...
The coast between Barcelona and France is also stunning!!