Al,
I do think XAO is worth more, however the potential for Mumb is better. I think on equal footing you may stand a chance. However, Im not sure how other people feel about domain swapping... I get offers quite frequently to do it, but the deals are never worth it for me. The only time I would accept a swap, is if I knew I would collect substancially more value than the current name.
I think the owner will decline your offer...thats just my guess.
Justin