If I *had* to buy a Titan with the money, it'd be a Reaver. The Warlord is just too big, and the Warhound isn't significantly better than a baneblade, which is far cheaper to purchase and much more accessable for the casual consumer.
However, if the only qualification was that I spend the money on GW product, but was given enough to buy a Reaver, I'd get some forge world hardware for my Space Marines in the form of a Chaplain dreadnought, a couple of venerable MkIV dreadnoughts with assorted arms, ten sets of reinforced extra armour, and the MkIVc Predator. I'd also load up on the MkIV armour kits labelled as Red Scorpions Veterans. Old armour is better armour, after all. I'd take what was left over from that (as all of that totals around 250 pounds, combined) and order up a Space Marine Battle Company, a Vindicator Linebreaker Squadron, and a couple Black Reach boxes.
IN THE US? ASK ME HOW TO GET 10% OFF GAMES WORKSHOP PRODUCTS!