Optically the new lens is superior to the 100-300 APO (see, for example, David K's review in Photoworld, which might also be posted at the photoclub alpha site. Photozone's review also suggests the new lens may be or is the best optically in its class. But, other than being more expensive, the new lens also is much larger and heavier. This may or may not make a difference to you, or other's considering getting it. The new lens has SSM focusing which is, to my knowledge, faster (perhaps considerably faster) than the older APO lens.
Are the improvements in the new lens worth the added expense, and size and weight? That's a personal decision nobody else can make. If you don't plan on going larger in print size than perhaps 8x10 you might never see the optical differences (unless you also do significant cropping of an image to get a portion of the original to 8x10). What MP camera are you using, or planning to use in the future? The higher the MP, and the sensor's resolution, the more you may see the optical differences.
Lots of things to consider. If it was me? The new lens without a doubt.
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Mark Van Bergh