In my opinion, the 3 main and strong advantages of platinium are chess mentor, chess tv and chess videos. If you can afford it, and have the time to use those sources, I think it's very much worth it.

Later correction: NOT platinium but diamond.

Quid unquam Romanorum nobis?

Well yes obviously the roads... the roads go without saying. But apart from the aqueduct, the sanitation and the roads...

Go with the Gold if you decide to upgrade whatever you paid will be pro-rated and credited towards upgrading. Once you pay for a more expensive option I don't think you can downgrade your account in case you don't have the time or inclination to use all the extra features, most of which you can get samples of as a Gold Member. Then you can decide if you want to further upgrade which involves spending more money.

 you don't get the Mentor or the Videos for Platinum, you need Diamond.

Oops, you're right, my mistake. I meant to say diamond.