Their Basically the same except for the Exhaust side and the wastegate setup. The HX35 has an 11 or 12 cm Exhaust housing where the HY35 has a 9 cm exhaust housing. The wastegate setup on the HY is not modifyable so you have to use a turnbuckle to bind the wastegate shut to get more boost. The HX35 you can get a modified boost elbow for getting more boost.
Hy35 max boost is about 32psi where the HX35 is more like 35-38psi.
I would never install an HY35 when a good HX35 is available. Although the HY35 is stock for 01 automatics, it doesn't have the same airflow/HP potential of an HX35.