I disagree with everyone here. I don't recommend either and I will tell you why- both white and silver fillings have mercury. The white has a little less, true, but it also has BPA.
I know, I just made your decision that much harder, but let me ask you this- why are you filling this tooth? Is it because it is causing her pain or is it because the dentist found a cavity on an x-ray/cleaning and told you you should?
If the tooth is NOT causing her pain, I recommend not filling it, especially if it is a baby tooth.
There are many, many reasons why she could have a cavity- some of us are genetically predisposed, some of us have more acidic saliva, or problems with our parotid glands, or even the shape of the tooth or placement of the tooth can contribute (molars in particular tend to hold food even if we brush really well.)
I would leave it alone and change your diet. Buy the book Nourishing Traditions and really read up on Weston A. Price studies about how to heal your teeth without fillings. Then RUN, don't walk, to get a water purifier that filters out fluoride.
It may sound psycho, but my daughter did not have enamel on her baby teeth at ALL when they came in. They were riddled with cavities. I struggled over what to do with this 9 month old with terrible teeth- the options were to strap her down and fill them or sedate her and fill them and I was scared to death to do either one. My gut told me there was a better way, and I did research until I was blue in the face and it paid off. Her teeth do look terrible- it's true- but they are HEALED and totally healthy now, AND she is actually growing enamel on her teeth. They are hard as little rocks and she has never had an ounce of pain from them.
We did a diet change, brush diligently twice a day and use MI paste (vanilla, and not "plus"- you can buy it on eBay or through holistic dentists,) give her Hyland's Bioplasma tablets (buy them through drugstore.com- they last forever,) and she has gotten 3 ozone treatments downtown with Dr. Stephen Gordon.
I recommend doing a LOT of reading before you decide what to do- dental work can affect you systemically more than you know. I am 33 and have a mouth full of silver fillings as well as white- 2 have broken and I am just now realizing I have mercury toxicity, and my children probably have it too from ME and my mouth :( It might seem like "one little filling- what's the big deal" but please, please read up and really find a dentist who does integrative dentistry. It costs a LOT, but it is THAT important!!! I do agree when others say don't beat yourself up over her having a cavity, but please don't put chemicals in her that might poison her or cause immune problems or even problems in HER kids later on- that is worth beating yourself up over!!
Here is what I wrote about my daughter's teeth (she is now 3) awhile back, if you are curious.
http://chickiepea.wordpress.com/2010/06/01/healing-caviti...
Okay, stepping off my fruit-cake sounding soapbox now! :)
M.