I have done all the routes a few times, in lorries cars and vans, with my final destination being just outside Elche, and I found the best route without a doubt is the Santander Burgos, Madrid(now the Mwayring road is open Chargeable!!)Alicante the best ( now at Elche there is a new Bypass that takes you up to just south of Benidorm saving even more time to Javea etc (that is chargeable and not on TomTom yet! even with mapshare and i have reported a few times to get it updated). I think you can also divert just before Vienna to to get to Calpe also, but I dont know that route well.
I choose Santander over Bilbao, as the Brittany Ferries, are better boats, more modern and comfortable (saying that I havent been on P&) for years so that may be outdated info).
If anybody is needing to drive over on a frequent basis I would recommend the Brittany Ferries, Property owners abroad club, cost about 125? to join but you can make that back on the first trip at the right time, but the discounts on future trips and the 10% off all meals etc, free breaksfast, priority boarding) and upgrades are well worth the investment ( you need to do at least two return trips a year for it to payback nicely.