Soy Formula - Need opinions.

My baby has been suffering from an upset tummy and constipation. She is on Similac Total Comfort. I called the doctor and they recommended Similac Alimentum. I spoke with my mom about it and turns out I had a milk allergy as a baby and had to be on a soy formula and nothing else worked. So here's my question. Do I try the Similac Soy since it worked for me as a baby? Or just try the Alimentum. I'd really like to avoid the alimentum if possible because it costs a ton more, but I don't know which to try.