NAS selection

I am trying to decide which NAS to purchase. The two I am looking at are the Synology DS509 Scalable 5-bay SATA NAS and the other is the QNAP TS-639 Pro Network Storage. I plan to fill the devices with 1.5tb harddrives, running raid 5 or 6.

I like the fact that the DS509 has a DX5 5-bay Expansion Solution for DS509. I also enjoy that it has a built in mail server and supports 2tb drives. This compares to the TS-639 that it supports 1.5tb drives (at least as far as i know).

Any thoughts would be appriecated.

Thanks

Apparently the

QNAP Systems, Inc. announced the availability of XDove an all-in-one feature rich mail server for its TS-109/ 209/ 409/ 509/ 639 Turbo NAS series. I found directions at http://wiki.qnap.com/wiki/XDove_Configuration

Therefore the only reason I am still concirdering the synology is the DX5 expansion soultion.

Any ideas on an expansion solution for te TS-639?