If you are able to go down to 37mm, the Saxonia manual offers excellent value for money and is timelessly elegant. The Saxonia ultra thin is a no-no for the following reasons:
1. Although it is only 5.9mm high, the 40mm case with no seconds dial makes the watch look larger than it the other watches with a seconds hand.
2 The winding mechanism as pointed out earlier is less smooth by Lange high standards. I have the new 1815 up and down which has a similar problem due to the longer power reserve on one barrel. I have to wind my watch in excess of 150 turns to fully utilise the 72 hour power reserve.
The Saxonia Automatic at 38.5mm and 7.8mm thick is also a good looking watch but as commented before, the rotor takes up half the movement as compared to the discontinued saxo mat. The question is are you comfortable buying used. Some people are not I understand.
Which brings us to the new 1815 manual in 38.5mm. A very nice watch and arguably better proportioned than the 1815 40mm depending of course on your wrist size. Many have said that the 1815 is a superior watch to the Saxonia line and in turn the Richard Lange range is superior to the 1815 range. If you look at the catalogue, you will notice that more parts go into the equivalent Richard Lange as compared to the 1815 and again more parts would go into the 1815 manual than a saxonia manual. The Lange proprietary balance spring is used in the Richard Lange but not in the Saxonia. I am not sure about the 1815 manual but I know my 1815 up and down uses the Lange balance spring. What difference does this make? Very little in terms of accuracy as Lange watches in general have a tolerance of +3 -2 (similar to Patek) irrespective of the balance spring used but pyschologically, one may prefer to have a Lange with a manufactured by Lange balance spring.
In conclusion, buy a saxonia manual as you get much better value for money than the 1815 manual but if 37mm is too small, buy the 1815 manual.
Good luck in your purchase. Prices in the far east go up on 1 April so there are some great bargains around with the relatively strong sterling and euro