I guess I am disagreeing a little with the previous two respondents. The 104 is great, but I expect that any of the pros will tell you that they do quite a few head gasket replacements on 104's and not too many on 103's. The other side of the coin is the valve seal problem with the 103.
What the previous two respondents may not have picked up on in your post, is that you want best durability and economy. I think the 103 has a better chance of delivering that IF the valve seals have been upgraded and the engine uses no oil.
Either one will have risks. I'm sure that not ALL 104's will need a headgasket in their lifetime, and not ALL 103's will have valve seal problems.
And, either one would be a great car to have. The 124 is a great automobile with any engine. I've never driven a 200 like you see so many of on the roads in Germany, but I'll bet they are also a great driving car.
Best of luck,
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in