If you want to access the data simultaneously on more than one computers(doesn't matter they are Macs or PCs), NAS is your best option.
And this is especially true when you want to use both Macs and PCs. Because the only file system that can be used under both OS X and Windows is FAT32. But it won't allow files size larger than 4G. Depending on your needs, this can be a annoying problem. NAS doesn't have this problem. It's just a file server, not a hard drive.
RAID is for faster speed or added reliability. You don't need it to access the files.
EOS 40D+BG-EN3/350D/17-40 f4L/70-200 f4L IS/50 f1.8/18-55 f5.6/MacBook&ThinkPad