If you are not particular about resale as much and prefer a new watch any Omega Seamaster will be a good deal in the 10-15k range, plus will be NEW. An 8500 PO might be out of range but the 300m or Aquaterra are all under 15k.
Used Sub/GMT can only get the non-ceramic models which IMO not worth it. I have a Bond Seamaster and the clasp/bracelet is fantastic along with an all around solid feel. I was handling my dad's non-ceramic sub and it felt tinny to me with a really crappy clasp. I felt like the Seamaster was a better watch for less money.
I also have a GMT2c and it is a great improvement over the old GMT. The clasp is fantastic, and so is the bracelet and the overall feel of the watch.
At 10-15k I'd go Omega Seamaster. I would go 2nd hand Rolex GMT/Sub Ceramic at above 20k. I don't like the idea of paying 15-18k for an older model Rolex when I can get a new Seamaster.