It all depends on the mood and tone, do RnB you must be all over the board with how the scale falls depending on the key you do happy and sad songs, do something hard hitting hip hop, check back in most of the joints considered hardcore or classic its sad or minor, this also applied to some of the bands I played in, some of the harder stuff was always in a minor key, "using the black keys" LOL
just because its black doesnt make it a minor also...LOL("A" minor scale is all white keys) think of the application of what you are doing, study the mood you are setting it can't be a track that is too happy.......an emcee can hone in on that too they might not like to pick your joint up....good luck.
Here is an article I found on that subject too
http://guitar.about.com
Sometimes, the theme or mood you wish to create with a song doesn't suit the generally "happy" sounds that a major key tends to provide. In these situations, a minor key is often the best choice for your song.
Which isn't to say that a song written in a minor key has to be "sad", or that a song written in a major key need be "happy". There are thousands of songs written in major keys that certainly not uplifting (Ben Folds Five's "Brick" and Pink Floyd's "Wish You Were Here" are two examples), just as there are many tunes written in minor keys that reflect positive, happy feelings (like Dire Straits' "Sultans of Swing" or Santana's "Oye Como Va").
Many songwriters will use both major and minor keys within their songs, perhaps choosing a minor key for the verse, and a major key for the chorus, or vice versa. This has a nice effect, as it helps break up the monotony that sometimes results when a song lingers in one key. Often, when switching to a major key from a minor key, writers will choose to go to the Relative Major, which is three semitones up (or, on the guitar, three frets up) from the minor key the song is in. So, for example, if a song is in the key of E minor, the relative major of that key would be G major. Similarly, the Relative Minor of a major key is three semitones (or frets) down from that key; so if a song is in D major, it's relative minor key would be B minor.
We've got lots more to discuss, but before we do, we need to learn what chords we can use in a minor key.
We have a lot more chord choices when writing songs in minor keys than we do if we're writing in a major key. This is because we compile two scales to create these chord choices; both the (ascending version of the) melodic minor, and the aeolian (natural) minor scale.
It is not necessary to know or understand these scales in order to write good songs. What you need to summarize (and memorize) from the above illustration is when writing in a minor key, chords can be found starting on the root (minor), the 2nd (diminished or minor), the b3rd (major or augmented), the 4th (minor or major), the 5th (minor or major), the b6th (major), the 6th (diminished), the b7th (major), and the 7th(diminished) of the key you're in. So, when writing a song which stays in the key of E minor, we could use some or all of the following chords: Emin, F#dim, F#min, Gmaj, Gaug, Amin, Amaj, Bmin, Bmaj, Cmaj, C#dim, Dmaj, and D#dim.
Phew! Lots of stuff to worry and think about. You might want to keep this in mind too: in most "popular" music, diminished and augmented chords really don't get used a whole lot. So if the above list looks daunting, try sticking to the plain major and minor chords for now.
In many traditional harmony books, you'll see the above series of chords, accompanied by a diagram that illustrates "acceptable" progressions of these series of chords (eg. V chord can go to i, or to bVI, etc). I have chosen not to include such a list, as I find it to be rather restrictive. Try combining various chords from the above illustration of the chords in a minor key, and decide for yourself which sequences you do, and don't like, and develop your own "rules".
Now that we've learned what the diatonic chords in a minor key are, let's take some time to study a few tunes, and analyze them using this knowledge.
Here's a tune with a relatively simple chord progression; Peter Green's Black Magic Woman (made famous by Santana):
Dmin - Amin - Dmin - Gmin - Dmin - *Amin* - Dmin
* OFTEN PLAYED AS Amaj
All of the chords (including the Amaj possibility) fit into the key of D minor (which contains the chords Dmin, Edim, Emin, Fmaj, Gmin, Gmaj, Amin, Amaj, Bbmaj, Bdim, Cmaj, and C#dim). If we analyze Black Magic Woman numerically, we come up with i - v - i - iv - i - v(or V) - i. There are just a few simple chords here, but the tune is very effective, and it should be a great reminder that your song doesn't have to contain ten different chords to be great.
Next, we'll look at something slightly more complex. Most people will recognize the very famous Eagles tune Hotel California. Here are the chords for the intro and verse of the song:
Bmin - F#maj - Amaj - Emaj - Gmaj - Dmaj - Emin - F#maj
By studying the above progression, we'll can surmise that the song is in the key of B minor (which contains the chords Bmin, C#dim, C#min, Dmaj, Daug, Emin, Emaj, F#min, F#maj, Gmaj, G#dim, Amaj, A#dim). Knowing this, we can numerically represent the chord progression of the song as i - v - bVII - IV - bVI - bIII - iv - V in that key. Hotel California is a great illustration of a tune which more fully takes advantage of all the chords available in a minor key.
These were just a few examples of the millions of songs written in minor keys. To more fully comprehend minor keys, and how to write songs in minor keys, I highly reccomend analyzing dozens more tunes, in the same manner as illustrated above, until you get a better idea of what chord movements sound best to you, etc. Try "borrowing" parts of chord progressions from songs you like, and adapting them into your own songs. It will undoubtedly take a while to get used to doing this, but your efforts should pay off in no time, and you'll find yourself writing better and better chord progressions for your original songs. Good luck...