For a start, if you are considering either Bulgaria or Spain, you should not be thinking of serious investment but more of a holiday home for your own use. Having said that, Spain wins on several counts.
1. You will get far more use from a holiday home in Spain; the weather is better and there is practically year round sunshine in many areas, compared to a couple of months in Bulgaria.
2. In Spain, you are generally (although not always) buying at local prices. The majority of holiday home buyers in Spain right now are Spaniards, and if you do your homework you will buy at these rates.
3. The right areas in Spain will always give you a return over time, assuming that you bought good properties in the right locations. I have owned several nice homes in Spain over many years (still do), and always made healthy profits when reselling.
4. Certain areas on the Spanish Costas can give good long-term rental returns to the expat community. I know of buyers who recently paid around 200k for furnished apartments (resales) and are achieving 1,000 euro per month from long term tenants (expats on 2 or 3 year work contracts). Avoid long term lets to locals, they can acquire rights that are hard to shift.
5. Rentals in the majority of new build Bulgarian coastal and ski property are imposible to achieve, or are tiny and give around zero return after costs.
Things to watch in Spain: High community charges in some developments, and wrong price for the location (check adjoining developments). Never buy off-plan in the Costa del Sol -- lots of resales cheaper and often furnished to boot.
On a final note, the oft-mentioned issue of planning problems in Marbella is actually not a cause for concern in most cases, but with a couple of notable exceptions. Use a good local lawyer who knows the ground -- don't even think of using Irish or UK lawyers to do a conveyance in Spain.