Thanks for all the advice everyone, had a great time! I actually ended up staying in the town of Esslingen on the periphery of Stuttgart, a really beautiful old place with great pubs, beer and people

. The swim in the old art deco swimming pool fed by springs was great too, highly recommended.
The museums were great, I got to the Porsche museum about 9.30 and it already looked busy, but I was glad to find that most of the people were in large groups, so all I had to do was wait until they moved on to the next car and I had all the time and space I needed. Wonderful place and I spent 3hrs there examining all the cars in great detail and photographing. When you leaned into the old race cars they had been left with all the dents and scrapes and still smelled of oil and petrol, fantastic.
I ended up having plenty of time so decided I'd do the Mercedes museum the same day and was pleased to find it eerily empty, there were no more than 10 people in the entire building and I quite often found myself having an entire floor to myself which was great, I spent ages just going round each car, my favourites being the gullwing SL and the superb, swoopy red 500 roadster from the 30s. You don't realise just how massive some of these cars are til you walk round them. Again, a thoroughly enjoyable experience, though the staff weren't as helpful or friendly as in the Porsche museum, apart from the lovely girl in the coffee shop. Again, 3 hrs so it's certainly viable to do both in a day, I really would have struggled to use more time, though I didn't do factory tours at either plant.
I'll post flickr links to the 100s of pics I took when I get round to it rather than flood the forum, but in the meantime how about this baby :-D What you can't see in the photo are badges on the front and rear, no not AMG....... Maserati!