VLB cards will give you 32-bit access for IDE, while the ISA cards are only going to be 16-bit.

Some VLB cards only have a single IDE channel on VLB, while the other channel is on ISA.

We had a couple of VLB caching controllers, and from what I remember it meant that small writes to disk (e.g. creation of temporary files, unzipping a bunch of .txt files) could be written to the card at the VLB speed (wikipedia tells me this would have been up to 133MB/s for the 486DX33 we had), rather than the ISA speed (16MB/s?) or whatever the hard drive write speed was, freeing up the CPU to then carry on with other things whilst the controller handled writing the cache out to the slower hard drive. Risk of data loss if there was a power cut, but they were pretty rare and we unplugged the computer during thunderstorms.

Given that hard drives weren't all that fast (<10MB/s?), I don't know how much difference a non-caching VLB controller would have made over an ISA controller. There might have been something about VLB allowing direct access to system RAM, faster than the ISA DMA. I'm sure someone around here will know the technical details and will have benchmarked the differences.

First card we had was a Tekram DC680t, which I think I managed to break after temporarily moving it to a friends computer (or at least, it wouldn't boot when I moved it back). The replacement was a Promise DC4030VL-2 which gave no problems at all.

There might have been something about VLB allowing direct access to system RAM, faster than the ISA DMA

Typical I/O cards don't have DMA capabilities, only PIO. So real difference between such cards is rather small.