You cannot really compare The Pearl and Zig Zag. Those are two different projects. Zig Zag is cheaper, but they have less facilities. The Pearl is like buying a lifestyle, living in a 5* hotel where you have an ownership for your part.
The thing about The Pearl is the exclusivity-you cannot go in and out unless you live/work there or someone invited you. Only those who live in there can go to the schools in The Pearl, etc.

If you actually live outside Qatar and want to buy it only for vacation- why not? You can rent it out during the year and when you want to come for vacation, you still have you place and there is no need for you to wait for visa, since you get the RP once your property is ready.

For the actual rent prediction... in my opinion, you can only see it clearly when some people would actually live there and at least a part of the project would be completed.

If you're looking for a problem, you're probably gonna find one.