Their both the same machine, same parts but with a different cyclone assembly.
I prefer the 14 from a useability point of view, find it easier to push and the handle arrangement is slightly better but thats about it.
However, if you feel you would need to clean the cyclone out alot then I'd say a DC07, as its easier to internally clean the cyclone.
The filters are the same, but a different size, so you have to buy specific DC14 & 15 ones. Clutches and motors are the same as DC04, 07 and 14, so no worries if you have those spares
DC14 feels lighter to me, but I think thats more due to the shape and centre of gravity than any weight difference, but I've not weighed either machine!
I wrote up one of my DC07 Stripdowns - http://www.vacuumland.org/cgi-bin/TD/TD-VIEWTHREAD.cgi?21791
if it helps you decide. Not created a DC14 how-to yet but its pretty much exactly the same!