The 40 will give you some more room to grow, the 35 is only a little better than the H1C.
RIP 91.5 W250 5 speed. The great experiment. 92 W250 HD 47RH with Compushift. Upgraded H1C to 62/60/16, Scheid Lightning VE, 60# valve springs, DAP 7X.010 injectors, 4" DE exhaust, home made cooler tubes. 01 2500 "the work truck".
HX40 also comes with the bonus of a 4" exhaust outlet. Little extra fab involved in it's installation, but it's all around better if you're willing to put in the work.
1990 W250 4x4 Rattles, growls, whistles, and whines.