I think it would depend on the kind of storage and power consumption you are looking at. If you are looking at somewhere around 500GB - 1 TB, then the Router + External HDD Combo should suffice in terms of storage plus power consumption. But if you are looking in excess of 1 TB then i would say go for a E-Trayz setup.
@edrill - This is just my personal opinion, but from the very first day i felt that DD-WRT was the most flawed firmware ever built. I still remember flashing it on my old WRT54GL's and ending up with a lot of grief. But i guess that wasnt enough so i revisited the grief by trying it on the ASUS router. Finally turned to tomato for better stability and peace of mind

. So what if i cant use the torrent client atleast the nas combo plus stability is far more convenient. I think ill just wait till these guys build a torrent client for the tomato, because it still remains the most rock solid firmware. PERIOD.
Edit: Apparently today i was browsing the linksysinfo forum and there is a new twist in the tale. Optware installation is possible even through the tomato firmware.
Tomato ND USB Mod - LinksysInfo - Community Forums for Linksys Devices
With USB enabled you can install Optware and additional software packages. Follow instructions in
this post, and in
DD-WRT Optware Wiki to install Optware (but do not modify search PATH or LD_LIBRARY_PATH as dd-wrt wiki says). Standard builds of this firmware do not include tools to partition USB drives, or to format them using Ext2/Ext3 Linux native filesystems. To do that you either need a Linux box, or you can use the set of utilities prepared by
ray123 and
posted here. Starting from v 20, the set of Ext2/Ext3 filesystem utilities is also included into the "Extras" build.
The possibilities are endless with optware. Torrent, rapidshare, mediaserver, and lots of more add-ons to add some icing on the cake. But the only constraint being time, which means that this experiment would take a while

.