There are many types of infant formula available, but the two most common are cow's milk based and soy formula. Soy milk products have certainly gotten a great deal of attention as a replacement for dairy products in recent years. There are those who advocate soy protein as being "all natural" and generally good for you. However, parents shouldn't be so hasty to begin their child on soy formula or switch them to soy because of minor problems with cow's milk based formula.
What the American Academy of Pediatrics Says
Cow's milk based formula remains the closest to breastmilk, which is known to be the best source of nutrition for babies. Because of that, the AAP stance is that all healthy babies should begin on cow's milk based formula. Soy milk should be the backup for babies who show difficulty digesting cow's milk formula or for families who follow a vegetarian diet.
What about generics, DHA, and the like? For more info see: Choosing Baby Formula
Using Soy Unnecessarily
Many parents take one or two possible signs of difficulty with cow's milk based formula as an indication to switch to soy infant formula. But that really may not be necessary or beneficial.
First, most babies do spit up, some quite a bit. But spitting up doesn't necessarily mean that they aren't tolerating cow's milk formula. Sometimes spitting up is more of a laundry problem than a medical one.
Soy formula also should not be used as a method to treat colic or general fussiness unless recommended by a doctor. Your doctor may point you in a completely different direction, such as a hypoallergenic formula.
Don't Stock Up too Soon on What You Think Is the Best Infant Formula
Some thrifty, expectant parents may be tempted to "stock up" on formula when good deals or coupons come their way. In the end, they may find that they are actually wasting money because when baby finally arrives, he just cannot tolerate that type of formula.
The bottom line: for healthy infants being fed formula, cow's milk is the best starting point, only switch to soy or another type of formula based on a doctor's recommendation.
See also: Water Guidelines for Baby Formula.