Personally, I would rather watch a rerun of Buffy than 24. 24 has just not held my interest as much as I hoped. (DISCLAIMER: I am addicted to Buffy...I will be watching about 5 hours worth when I get home).
You might not want these two episodes spoiled out of order for you, since they come a few episodes after the start of season 6. Everyone knows I love Buffy to death, but 24 might be easier to follow. You can always catch season 6 in order on UPN this summer, after you've watched seasons 1-5 on FX this winter / spring.
I'd watch Buffy. These episodes aren't available in syndication yet, and 24 is repeated at least 3 times before next week.
If you don't have FX, catch a rerun of 24 at 9 pm on Friday.
As a recent Buffy convert, I'm excited because tonight's 8 pm repeat is the ONLY episode I haven't seen yet.
I only started watching FX reruns in September and started in on the new season around October or so. I'll be happy to complete my viewing this evening