These are entirely different classes of travel. Portugal+Spain is an easy trip within the usual Western world, close to home and without any significant apparent dangers other than the usual pickpockets and the expensive Euro zone. You can easily get home if needed (regular flights from Madrid and Barcelona directly to TLV, connections through all around Europe from everywhere else).
Russia, Cuba and China are different kinds of animals.
Russia is an unstable place, other than in the central cities (Moscow, St. Petersburg, provincial capitals, etc) you are in danger. If you go too much south, you'll get to the Muslim areas (Chechnya, Dagestan, Azeri border) where being an Israeli might not be healthy (even relatively to the rest of Russia. They're not big fans). You might not speak the language, you would definitely not know the local way of doing business, and you'll be definitely paying more than a Russian-speaker for everything. Much more. If you stay close to the central places you would be relatively safe, but there were Israelis coming home without fingers from Russia, as you probably know. (FYI for all the haters - lived in Moscow for several years, I know what's going on there from my own experience)
China - same about the language and the cost difference, but (IMHO) much safer for foreign travelers. English spoken in the big cities, talk to younger people (accent may be horrible though...)
Cuba - FAAAAAAAAR. If you go to European parts of Russia, or Portugal+Spain - you can go for a week or two. Going all the way to Cuba (or China) for a week - a huge waste. If you're going for a long time though it might be very interesting, and I would suggest Cuba as the best candidate. It is very unique, and will, likely, disappear (in its current shape) in a decade or two. Once the Castro's are gone, my guess is that Cuba will get back to the close ties with the US it had before, and will change significantly.