Still get the free games from the PSN Welcome Back promotion

You might not like this answer, but if you didn't have a PSN account in April, you won't qualify for the freebies. If you have only just purchased your PS3, then this will be the case, and you should just keep the disc.

On the other hand, if you do qualify for the freebies, then yes--the games should share the data. I base this on the example of Dead Space: Extraction. I had the game on PSN before I purchased Dead Space 2 Collectors Edition, which came with a copy of the game on the disc. I installed Extraction from the disc, and then launched it from the disc--it shared the data from my previous PSN version of the game.

Of course, it could just be a special case with Extraction, but that would be quite unexpected.

You can have two different memory cards, one for each user, but you will need to do a factory reset each time you swtich your memory cards, see the following video, courtesy of IGN: