[xsl] xsl:function vs named templates (Which is better?)

["Jernigan, TieSha" <jernigant@xxxxxxxxxxxxxxx>]

So I have a fairly general question. I am using XSLT 2.0 and am
interested in creating user defined functions. Which construct is better
to use... xsl:function or a named template?

An example of what I am trying to do with the user defined function is
below.

So for example if I have

<greater>freq</greater>
<lesser>freq</lesser>

I want to replace it with .....

<greater>upperfreq</greater>
<lesser>lowerfreq</lesser>

Thanks
  TieSha