I'd agree with Mark.

I'd love to have the Mk II your daughter is looking at but the Mk I is still so good I can't see why I need to replace it just yet.

If she has the money to spend she will not regret buying the Canon, I can't say the same will be true for the Sigma. There is also a very good chance that the Canon will holds it's value really well. I bought my 70-200 3 years ago, it's worth more now that it was then, even though it's had some really hard professional use!

Cheers

Cris