Both RSS and SQL approaches will work, but SQL is faster. I mean if you import RSS data all your posts and pages will be assigned to only one language(as true for importer version 0.5) , means you cannot map pages and posts to different languages(would be nice feature of import plugin). Also apart from page or post knowing their own language(“Language of this page” field) you will loose pages translations interconnections(“This is a translation of” field).
With SQL approach you keep all data safe. Once you exported all your data into SQL file you need to replace all hardcoded domain names to the domain names you are going to move your data in. As soon as all plugins and wordpress versions equal on both domains you should not experience any inconsistency or data loss while migrating.
But all this should work only for “from wordpress to wordpress” migration. If you are doing something like custom CMS to wordpress migration. You should stick to SQL approach and create sql import script that will represent you custom CMS data in WPML schema way. Yes you need to research WPML DB schema in detail to do that. My googling abilities didn’t give any result on any software to generate that SQL script for you.
