I can't vouch for the weather as I've only been in the autumn, but personally I'd rather go to Siurana for a week. There are far more routes and a wider variety of styles of climbing. Margalef is generally shorter routes all on conglomerate so it's a lot of pockets and a lot of pebble pulling. Siurana is a more traditional limestone venue, plenty of slabs and steep stuff and everything inbetween. The camping is great too, but watch out for the owner Toni - he's a little weird.