Perhaps they could be interchangeable but one is a word the other is not. So I consulted my favorite English major~Garrison Keillor. No, not really, he still hasn't responded to my postcard. Oh well, I went for the next best~Yahoo Answers.
For most, unclutter is a word, declutter is not. However, within the minimalist movement, consensus seems to go with declutter.
Verb--> You need to declutter to leave it uncluttered.
Consensus has it, we're decluttering .