I've used both Diamond D and 199 and like both. The only difference I can observe as a tech is in packing and the look and shade. Finishing is about the same. I have not use Diamond D enough to gauge its durability(frequency of repairs and color stability) against 199.
from the numbers have read about, impact strength is greater on Diamond D with studies using notched Izod tests. (over .8 for Diamond D and under .6 with 199)
this was from the Journal of prosthetic Dentistry in 2008:
Mean static flexure strength (SDs) in MPa were: Eclipse, 127.11 (5.83); Diamond D, 84.92 (5.10); Lucitone 199, 83.96 (11.96); HI-I, 79.54 (5.84); and Nature-Cryl Hi-Plus, 75.82 (6.96).
Mean flexural strengths (SDs) in MPa postcycling were: Eclipse, 113.36 (31.29); Diamond D, 88.26 (5.46); Nature-Cryl Hi-Plus, 81.86 (4.93); HI-I, 79.18 (6.60); and Lucitone 199, 74.34 (4.95).
...close to the same numbers here on flexural strength but diamond d did out perform 199 a bit in this study, So unless someone can sway me otherwise because of something im missing here, id say Diamond D is the way to go. It costs less plus it has a cooler name! Go big D... lol !!!!