I'm quite impressed with Argos value drills these days. They've got entirely metal gears and the motor shaft has a flat on it, with a pinion gear that has a matching profile, meaning the gear won't come loose and slip around the motor shaft (a problem I've experienced before). Plus the casing is decent.

Not had a look at the Tesco drills - there are none in my local store - but other value makes such as B&Q's range isn't as good as the Argos drills in my opinion.