Well..Connectix VGS actually plays a majority of playstation games (my estimate is over 70%) without major problems, while Bleem plays only about 15-20% under the same criteria. However, VGS does not really enhance the game other than anti-aliasing the whole final image. Bleem on the other hand can support D3D with high-res and filtering.
Therefore, if you don't have a PSX and want to play PSX games, use VGS. If you already have a PSX and want to see some of your favorite games in high-res (provided that they work), get bleem.
Bleem also hasn't been updated in ages - since they began working on the DC version. Rumor has it that they plan on distributing a new PC version like the DreamCast version (in multiple sets each with its own playable games). This of course means you might have to pay for Bleem again.