Personally, I'd rather buy an external HD and an Airport Extreme. It'll work out to be the same, and you don't have 2 devices in one, which just gives you more options if your HD gets full, or you buy 2 separate harddisks, or the wireless malfunctions (covered under warranty, but still).
Also, I'd get to choose my HD if I purchased an external, and I like Seagate harddisks, and don't want a Western Digital (WD) or Maxtor harddisk. On that note, I do own a WD Passport 2.5" portable harddisk, though. (I don't think the reliability of their full sized drives is tied to their portable laptop drives.) Also, I wouldn't buy a Lacie because they just put ANY harddisk in there. They buy brand name HDs, but the actual brand you get in there is whatever is cheapest for them to buy at the time. It could be a Maxtor.