I would recommend the 2.8 that is quite rapid, nice to drive and quite good on petrol consumption and insurance. There is also a decent supply of them available, and as others have suggested, best to buy one from an enthusiast. Apart from my 2.8, I also have an s54M, and I would say that the difference between them is like having a prancing pony and a racehorse, one is fun and practical, forgiving, and can go at a good gallop; the other is fast, needs much more care and handling, and has to be raced to get the best out of it, or it can get temperamental.
Whichever one you end up with, it will absolutely put a smile on your face!!