If you look at the D500 on The Link's website you may still be able to their their marvelous offer of the D500 plus the Samsung Photo Printer for the princely sum of £129.99 - the printer alone is worth around £60, so that is a marvelous offer.
I'd go for the D500 myself also, it is a very well established bug-free handset that does precisely what it says on the tin
which is a bit of a change these days.
The Z500 can be had on vodafone for around £130 on PAYG, completely free on contract (but then so can a lot of others on contract!).