If it is to record SKY programs then get SKY+. If you are talking about recording General Freeview programs then get a twin tuner PVR/DVDR.
Both Twin tuner boxes and SKY+ allow you to record one program whilst watching another.
Any system that allows you to record one channel whilst watching another requires that it has twin tuners.
PVR's have twin tuners, but cannot record sky, they can only record freeview via UHF aerial.
DVDR's never have twin tuners.... but could record from sky. However there would be no alternative but to watch that sky channel.
It would be possible to record a freeview channel on a DVDR whilst watching a sky channel.
Unless you have two independent sky decoders, sky+ is the only system that will permit recording one sky channel whilst watching another.
It is also the simplest system to use.