Have a 7D. XTI or XSI as backup?

So, my daughter is asking for some advice. See the quote below. I'm contemplating asking for my XTI back but want to see if you all think she should keep both the XTI & XSI as backups or, if she were to keep one, which one would you recommend? Thanks for your advice!

XSi.

Personally, I would stick with the XTi as a backup so she doesn't have to keep two different types of memory cards. I prefer CF cards so I'm quite happy to keep one of my XTi's as a backup to the new 7D I bought. Plus, the XSi will sell for more than the XTi.

I'd go with the XSi, but one type of memory card and the hackability of the XTi are both compelling arguments.

I'd say XTi for the CF card. And also if spot metering or ISO3200 is needed you can use a hacked card to enable those functions (and shutter count and different AF point patterns etc.).
I actually had an XTi as a backup with my 7D. (But I just sold the XTi because I found a deal on a T1i I couldn't refuse...)

i have a gripped xsi for sale if you're interested.

Thanks for the responses. Looks like she decided on sellng the XSI.
I really appreciate your opinions! Thanks again.