I've been through this exact decision process myself.
Last year my auntie was diagnosed with pancreatic cancer so i decided to do some fundraising for pancreatic cancer UK by running race for lifes etc in 2008. I've never run in my life and so it was a challenge for me
By coincidence I was doing a lejog this year, but for my own satisfaction as a cycle tourist. About 2 months before my lejog, I had an injury and my doc told me that on no account should I ever run on uneven surfaces, ie out of doors. This completely put pay to my running charity plans.
So for me it was a no brainer - so i didn't waste the opportunity to raise funds I did my lejog for pancreatic cancer UK, but also had a really fun tour. It was still really hard.
Although I felt funny about it at first what I found was that my non cycling family and friends were really impressed at the effort I made and gave me lots of money. A cycle tour is still a big deal to a lot of people, and so it made for some nice donations and a lot of support. I raised 2 grand in the end. It was really satisfying.
I don't think I'd do any old tour for any old charity. This one was completely personal to me, and has actually helped me deal with the grim realities of my auntie's illness. (For what it's worth, pancreatic cancer is pretty much a death sentence. Most people don't last a year from diagnosis and only 2% last five years. Bizarrely you wouldn't know my auntie was ill and she's still going strong!)