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As an owner of both a DS dictionary and an electronic dictionary, I'd have to say it depends what you're looking for. DS Dictionaries are good for your basic word and kanji lookups - certain ones have neat features like showing you the animated stroke order on kanji, etc.

Many of the mid-to-high range electronic dictionaries are a lot more comprehensive and have a lot more features, as well as a lot more entries in their dictionaries. They have more specialized dictionaries (technical, medical, science, etc), and often contain a lot more less-common kanji.

If you're a beginner Japanese student or just want some basic dictionary functionality, I'd just go for one of the many DS titles there are. If you're an intermediate/advanced learner or if you're studying abroad in Japan, I'd recommend an electronic dictionary. The Canon WordTank G55 is a great one for learners of Japanese, because the interface can be switched to English, and compared to other electronic dictionaries, it's not too expensive (roughly $230 last time I looked).