Hi, quite the question, I'm sure you'll get plenty of responses. Just my 2C worth. The Spanish gun market at this time is a bit weak in the resale area, but with that, who knows what the future will hold. Perhaps in a couple or more years the market will take an upswing. One of those things that no one can predict. An advantage with modern Spanish guns is if they come from a good maker, they will for the most part be a good pick up and go gun. Regardless of what some will say, Spanish guns can be gems, sure there are dogs out there, but overall the guns will be decent hunting guns if you don't abuse them and treat them kindly.
With vintage Brit guns, there is a whole set of issues if you will. Are the bbls good, is the gun in proof,any problems with the stock (old cracks, repairs oil soaked wood etc), has the gun been monkeyed with over the years and a host of other issues. A good Brit gun can be a joy to hold and use, the resale maybe an issue also. Most of the times a Brit gun will hold it's value, but if your looking at a good shooter, your not going to make a ton of money on resale. Right now there are a lot of Brit guns on the market and there will be for quite a while. You should get your investment back I would think. As I said, with a Brit gun, do your homework first, it will payoff. The one downside to Brit guns is you'll have to shoot low pressure loads in it. With Spanish guns, any off the shelf loads will be just fine.
As I said, just my 2c worth.
Good luck! Most of the fun is in the looking!