OT: Wengo/Skype/Gizmo/etc ... why?

[Paul Lemmons]

For those of you that use the SIP or Skype services this may be a stupid 
question. But why? Using any number of instant messaging clients I can 
talk to another person using that same client.

I am about to send my daughter overseas for a semester (Oxford) and she 
is accustomed to many of the usual IM clients. I am wrestling with what 
the best solution will be. She uses Windows XP and I use Kubuntu so it 
will need to be cross platform. Do I use an IM client or have her go 
through the learning curve of putting on Skype/SIP?

I am absolutely sure I am missing some glaring difference between the 
two services. Clearly I have never used either to any significant extent 
so if this is a *really* stupid question please be kind :)