Like Carmen says, I think what each person picks will depend on background and other choices. I don't think any one cause is necessarily any more worthy than another.
For myself, I knit burial gowns for babies. I help my daughter run a web site for those who want to do the same and would like to blog/visit together while doing so. Sort of a knit-along, but it's not limited to knitting.
Like you, I'm pulled in different directions and sometimes knit for other charities too. The preemie project, Warming Grace, and others that come along.
The burial gowns pull me most strongly because I feel it's an area many people don't want to think about or would prefer to ignore. (Not talking knitters here, but society in general). I guess I feel like the number of people willing to do this is more limited than some other areas.
Most parents aren't in any shape to go shopping for clothes to bury a baby and finding ones that fit preemies is very difficult. Plus, shopping in the baby section is excruciating for those who have lost a child. When a nurse brings their child to them dressed in a hand made outfit, it's my hope that just knowing someone cared enough to do that brings them a tiny bit of comfort.
Warning... very personal opinions below... feel free to skip.
It's amazing how often when people ask me what I'm knitting and discover what it is that I'm asked.. "but don't you find that morbid? How can you stand to work on something so sad?"
I tell them that I find it much sadder that families are left to grieve alone when a little kindness can make such a difference in their lives. The number of grieving parents who have told me that they have no one who they can talk to and how often their friends pull away is really heartbreaking.
The US hides from death in general. But people especially don't like to believe that babies die and that means parents are often left isolated and alone. They discover that few friends, if any, are able to listen to them talk about their child. It's a subject that is "off limits" It's ok to talk about our memories of grandma or grandpa, but not a child who died.
I think it's very difficult to value life and teach our children to value life until we stop hiding death. Death isn't real so why should teens worry about what happens when they drink and drive, deal with gangs, pull stupid deadly pranks, etc.
I find time whenever and wherever... I can fit yarn for a hat in my pocket while I flip pancakes. Knit while my husband pumps gas.
I don't think it matters if someone has time to knit a single little hat, 10 helmet liners, or 50 caps... each one counts and together they all add up. When I don't have time or I'm involved in something else that's fine. If we each just do what we can, then it all adds up!