Miles are worth more, but it depends on how the cancelled connecting flight inconvenienced you. Were you reaccomodated? Did you lose out on anything monetarily? If so, then I would go back and ask them to either offer both (given your loss) OR bump up the voucher to reflect a value closer to your loss. If not, then the miles should suffice.
Thanks for the info. I was reaccomodated on next flight. I'm fine with the 15K miles, but I thought their conversion for evoucher used to be better. I think in the past when I was offered miles/evoucher option, the evoucher option was better (e.g. 15K miles or $250 voucher). Does anybody have any experience with the evoucher-miles conversion rate if you're given an option?