Which key to buy?

I'm looking to move up from my C harp and I'm not sure which key to purchase
next, either A or D, for playing blues in E or A.  I'm looking at these two
keys mostly because I play acoustic guitar at the same time and blues
progressions in A and E are easier than most others.  Right now I capo at the
third fret and play in the key of G as if it where the key of E.  Anyways, my
question is: Besides being in different keys, how do these two harps differ in
playability?  I have heard that the A harps are real low and harder to play.
I'm still basically a beginner and play cross harp in the single note style.
If key is irrelavent, should I just go with the D harp, or is there some
special quality to the A harp.  I've never blown anything but a C harp. I will
probably get a Special 20, maybe a Marine Band, possibly a Lee Oskar.
                                                thanks for any help

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