I lean more toward using the set abbreviations. If I'm not mistaken, they are unique to each edition.
1 'A' Fireball
4 'DS' Fireball
4 '7th' Fireball
20 'M10' Fireball
4 'MM' Counterspell
8 'ZEN' Arid MesaI'm sure there is a table somewhere on the interwebs that would detail all abbreviations. On the other hand, there is no reason to not use abbreviations and long spellings.
Before I found deckbox.org, I began building my own site. One of the things that I was surprised at is the lack of a common abbreviation for set names. Or at least, I was unable to find an official list of abbreviations.
Some abbreviations can be determined by examining the HTML source from WotC's MTG Products page. For example, note the "SOM" abbreviation for "Scars of Mirrodin" in the src attribute of the img tag:
<td><i>Scars of Mirrodin</i> block</td>
<td><img class="article-image" src="http://www.wizards.com/Magic/Images/Expsym/exp_sym_SOM_C.gif" alt="Scars of Mirrodin" title="Scars of Mirrodin" border="0"></td>
<td>
<a href="/magic/tcg/products.aspx?x=mtg/tcg/products/scarsofmirrodin">
<i>Scars of Mirrodin</i>
</a>
</td>
However, this abbreviation isn't used for every set. Eg:
<td rowspan="3">
<i>Onslaught</i> block</td>
<td>
<img class="article-image" src="/magic/images/exp_sym_scourge.gif" alt="Scourge" title="Scourge" border="0">
</td>
<td>
<a href="/magic/tcg/productarticle.aspx?x=mtg_tcg_scourge_productinfo">
<i>Scourge</i>
</a>
</td>
My preference is to be as explicit as possible. Thus, I'd suggest sticking with the full set name.