I'd take the 210 any day. I find the size of the 124 to be just a bit tight, while the 210 is exactly right. The 210 has better seats, creaking seat frames notwithstanding. (The seats in the 124 are a bit small - too short on the bottom cushion, too short a backrest - and I'm only 6'0") The 210 has an air conditioner which stands up to Texas heat, the 124 does not. My 210 diesel is much faster, quieter, and delivers better fuel milage than my 124 diesel. In a crash, I'd rather be in a 210 any day.
Are there advantages of the 124 over the 210? Sure. The paint on early 124s is better - they have that nasty solvent based stuff as compared to the water based paint used as of 94/95. The earlier paint is more durable - but this problem affected the late 124 models as well. The 124 has an oil pressure guage. The early 124s were simpler than the 210, which is an advantage if you plan to drive it forever - but I don't plan to keep my cars that long. Frankly, I'm having trouble coming up with other areas in which the 124 is a better car than the 210.