Well let me note that they're only available in JP as far as I know.
Also the first one is on the DS while the second one, "7th Dragon 2020" is for the PSP. Storywise 2020 is actually a prequel to the DS title.
The DS one is actually quite challenging and somewhat of a pain when it comes to traveling on the world map. The major thing that comes up after the first part of the game is the "Flowr", poisonous flowers that cover the field that drains your HP when you step over them. And the World Map and Dungeons are FILLED with them. (part of the objective is to clear them from each area by defeating the bosses)
The second objective is to hunt down all the Dragons. There are about...600 or so of them? Though you only need to kill about 300 to complete the story. Many appear on the field, basically like the FOE in the Etrian Odyssey games.
The main part of the game is pretty straight forward. The bonus dungeon on the other hand is QUITE challenging. I've yet to even finish itand my characters are at Lvl.80 with the best equipment possible at that point. orz. The normal encounter enemies are powered up versions of the FOE-ish Dragons so...yeah. To think, all the boss class dragons come back even stronger too...orz
7th Dragon 2020 on the other hand is MUCH easier. I'm only having trouble defeating the final boss of the bonus dungeon in this one.
The cool part about 2020 is that you can choose your characters appearances from 10 styles(the DS version has 4character appearances for each class, just like Etrian Odyssey) and about 30+ VA voices. Its a pretty gorgeous cast. Though its just battle voices so, its not like they talk during the story. orz
Hope that info helps you in your decision. Oh btw, the DS version is under "SEGA'S the BEST" collection so you "should" be able to find it fairly cheap if you look for it.
