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Ferries, are better boats, more modern and comfortable (saying that I havent been on P&) for years so that may be outdated info).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.