There's no difference between the two types if you're just reading from the flash (assumed same or faster access time). But the problem is the write algorithm which the BIOS uses at startup to store some device infos. Take a look at the datasheet of the ST and SST flashs (different manufacturers!). They differ in sector size, manufacturer code, device code, algorithm etc...
If your BIOS can't handle this new type there seems to be no chance to get it running :-(
Mik