The DS212J that you have currently is just excruciatingly slow in my opinion. I don't have the best review and numbers in front of me on the differences http://www.andrewsreviews.co.uk/tm [andrewsreviews.co.uk]
but you can definitely see a general trend of speed with the better processors and larger amounts of RAM.
The DS213 is essentially the same as the DS212+ both having a 2Ghz Marvell ARM processor along with 512mb RAM. The DS213+ has a completely different dual core PPC 1.067GHz processor with free point calculations and a built in encryption chip. That's the technical jargon of the differences. The biggest change you would see in the DS213+ vs DS213 is in handling encrypted files. I read in a review that the DS213 almost goes to a crawl when trying to deal with many encrypted files. However, the DS213+ has a chip optimmized for that and performs quite admirably. This is very important for some people.
My biggest want from the DS213+ is the dual core processor and it's ability to better transcode video files (up to 1080p) to my various media hubs (HTPC, ipad, Android, etc). This is where the dual core processor helps a lot and it is said that the DS213+ should be able to run Plex with transcoding (perhaps not the best at 1080p).
But these new models come with many other features that are better than in DS212J too such as autoLAN on, USB 3.0, eSata, better power consumption, etc.
The main thing is that there is definitely a sale on the DS213+ model which makes it the same price as the DS213 and at that point, it's a no brainer for me to go with the DS213+. Hope that helps.