What is the best option to upgrade?

The easier of the two is adding RAM as there is a door at the bottom which you can access to plug in the new modules.

As far as storage how about either adding the SSD as a second drive if you don't use your optical drive. Here's how: Installing iMac Intel 21.5" EMC 2389 Dual Hard Drive This way allows you to use a smaller cheaper SSD as you aren't trying to use it for storage but more as a boot drive and to hold your apps. Then use your current HD to store your stuff (docs, picts, music...)

If your HD is to small or failing I would look at going with a hybrid drive (SSHD) these have a fast SSD cache yet also have deep storage of a traditional HD gaining the benefits of both technologies.

For myself I would add the RAM at least getting the system to 8 GB and add in the second drive (SSD here).