Although the part number remains the same, R-134a expansion valves have a different calibration than R-12 valves; this calibration difference can affect system performance. If you are converting from R-12 to R-134a, then by all means, switch to the R-134a expansion valve. Its inexpensive and will lessen the possibility of having to go back in for additional repairs due to an expansion valve leak.
If you have a W124 vehicle with R-12 and a leaky expansion valve and wish to stay with R-12, then you'll have to look aftermarket for an R-12 valve. M-B currently only provides R-134a calibrated expansion valves for the W124. I believe the same holds true for other models (e.g., W123, W126).
Does it make a difference? Depends on your ambient conditions. In the deep South, where M-B A/C systems can be easily overwhelmed by summer's heat and humidity, most definitely yes. I've been in R-12 vehicles equipped with R-134a calibrated exp. valves and A/C cooling was less than optimal. Subsequent installation of R-12 calibrated exp. valves greatly improved system performance.
[This message has been edited by jgl1 (edited 06-04-2000).]