Basically Intel make three types of "iCore" CPUs.
They are the i3 (the lowest), then the i5, then the i7 (top of range)
So an i5 CPU is going to be better than an i3 CPU so as a general rule go for an i5 over an i3 (though I have an i3 and it is fine). However over the next few years an i5 will deal better with video and other CPU intensive tasks than an i3.
Also the USB 2.0 spec has been replaced by the USB 3.0 spec (well over a year ago) so if you want to future proof then a desktop with USB 3.0 is better than one with USB 2.0.
Basically USB 3.0 will allow you to transfer data to say an external hard drive faster than USB 2.0.
I notice the two Lenovo from PC World only have USB 2.0 (though that may be a mistake on the PC World web site).
The Lenovo from John Lewis says it has 2 x USB 3.0 though these may be at the back of the case (and harder to get to), with the USB 2.0 at the front.
Be wary of small cases as if you ever want to add a card inside (a new graphics card) or extra hard disk there may not be room in the case.
Overall the PC from John Lewis is the best as it is a full size case, has a large 2Tb hard disk, an i5 CPU, and USB 3.0. But of course is it the most expensive.
However it is probably rather over specification for just "simple tasks" so you could probably get away with a lower spec (and cheaper) PC if you want to save money.
Note you don't get a screen with any of these PCs.
Any reason why you a limiting yourself to Lenovo?