A 2.5" HDD for a external caddy

I recently bought a caddy for a 2.5" HDD to use as an external hard drive for my laptop (to back up files, etc.).
I've been looking at Western Digital 250GB hard drives, but my quesiton is shall I go for:
http://www.amazon.co.uk/Western-Dig...CEPA/ref=sr_1_3?ie=UTF8&qid=1230910170&sr=8-3
or
http://www.amazon.co.uk/Western-Dig...8?ie=UTF8&s=electronics&qid=1230910110&sr=8-8

For £5 difference I'm tempted to go for the 16mb cache one, not for the 7200RPM (since the USB could/would be a bottleneck in data transfer) but for the cache size.
Could I have some opinions of which one to buy since I'm not sure at the moment. I would buy this one and exchange it for the one in my laptop (also 250GB) but I don't want to void my warranty...

 

Just take the cheapest one, the USB bus speed is too low for both of them, they will never reach the transfer speed they could have if plugged directly on the sata bus.

 

Ok, thanks for the reply!

Would you recommend http://www.wdc.com/en/products/products.asp?driveid=512 or http://www.seagate.com/ww/v/index.j...7eb4110VgnVCM100000f5ee0a0aRCRD&reqPage=Model ?

The western digital drive has a feature called "SecurePark", where the recording heads are parked off the disk during spin up/spin down/when it's off. Is this unique to WD or do Seagate have this and WD are 'hyping it up' so-to-speak?

 

I'm pretty sure all drives move their heads away from the platters when shut off properly. I think that's just marketing-speak from WD.

I would go with the 320GB WD Scorpio Blue personally, for around the same price as the 250GB models.