Only the vortex2 does A3D2, which is true positional sound. The MX200 might be OK, but its just one of dozens of cheap PCI cards that do A3D1. Nothing special there at all.

The MX200 is a good card and definitely worth it. I owned one back when they first came out and used it for games like Half-Life and other first person shooters.

The MX200 supports A3D 1.0 which does include positional audio. It works best with headphones.

A3D 2.0 which is what the MX300 supports does have more advanced 3D audio capabilities including occlusion modeling.

If you already own the MX200, stay with it. There aren't many games that support or took advantage of the A3D 2.0 vs. 1.0. And the Mx200 can do A3D 1.0 very well. Also, with AU8830 (Vortex 2) cards it is the same with 3DFX cards - they are very expensive, not to say overpriced, imo. So for Win 9x its great, but for DOS I can't recommend both cards, there are better alternatives in case of compatibility.

The MX200/Vortex 1 supports A3D 1.0 but no EAX support, Many other sound cards support both A3D 1.0 and EAX 1.0
The MX300/Vortex 2 supports A3D 2.0 and EAX 1.0, Only cards based on the Vortex 2 chip support A3d 2.0 which is why they are desired.

For compassion similar era SBLive! has support for A3D 1.0 and EAX 3

"Good for 98/DOS?"
Very much so! in Windows it can do some of its hardware mixing in hardware unlike lesser cards.
But Dos is really where it comes into its own. PCI card with good dos drivers and a Midi header. Thats a harad act to beat.

The EAX support on the Vortex 2 is only part of the buggy 2048 driver and is pretty awful.

MX200 is an interesting card from a historical standpoint. It's the expensive first gen DSP-based implementation of A3D. It received some software upgrades over its life. The Vortex ASIC was developed to make their cards far cheaper to build and get more OEMs onboard.