I currently have all 3 of the cars you mentioned, and have had *many* BMWs and Porsches in the last 19 years.
I think Porsche build quality is quite good, at least on the 911. On lesser models, it can be very, very bad (like the 924). I think the build quality on the 911 (I am talking pre-90 cars, don't know much about the newer ones) is the equal of most model MBs. I've never had reliability problems with my current 911, or any of the other Porsches I owned. No major repairs, no major cash outlays (other than voluntary, i.e. restoration). Parts are actually quite reasonable once you know where to get them, other than certain engine "hard parts" like pistons & cyls. etc. The 911 engine is very reliable (other than the smog year, 74-77), lots of people in the Porsche club that I know have 200,000 plus, some 300,000 plus, with the engine never apart.
My SC has 117,000 miles on it, and I have all the records since birth (EVERY one), and it has been an economical, reliable car.
I love BMWs, but I don't think their build quality is the greatest in the world. Fun cars, though.
As for restoring, I think the above is right, you need to restore the car you love. I don't think the make makes that much of a difference. I personally think the easist one to restore of all the cars you mention is the 911. It is very easy to drop the engine and trans, the interior is simple, it has a very simple body (no side moldings, etc. you can get it to a bare body in no time), and it overall is a very simple car (no radiator, super simple suspension, brakes etc.). All the 911s up to 89 were surprisingly straightforward cars.
As for restoring a BMW, there are a few I'd like to do. A '73 2002 tii is a really neat car, and a fully restored, stock tii is unusual and a lot of fun. The early coupes (the CS cars) are very pretty, but really rusted and can be a costly restore. I like owning/driving BMWs, but I must admit there are not many models that I lust after, enough to justify full blown, labor of love restoration.