Out of space

I download movies a lot and therefore I am nearly out of space in my hard disk.I have a 40 GB IDE hard disk and a 140 GB SATA one(which I use as my primary disk).Nearly 100 GB of the 180 GB has been taken up by movies so I have planned to buy an external hard disk.I went to Ritchie street today and enquired about the prices in a few shops there.I found that I could get a Seagate 100 GB at 2600 and 160 GB at 3100;these were the best prices I could see quoted there.Now I have to decide between the two and choose one.