The hardware spec. of these two models has a significant difference, since one is C2D 2.8GHz processor along w/ 2GB DDR2 RAM, and another is Celeron processor w/ 1GB DDR2 RAM. Besides, the TS-809U supports all 7 bonding (link aggregation) modes (which will be available since v3 firmware) and the other new enterprise features.
And if it's gonna be used for the database storage (probably thru iSCSI), then I think the performace would be one of the major considerations. And the hardware spec. will definitely make a hugh difference here.