Both GIA and AGS are considered very reliable grading labs. If you were going to split hairs, GIA certainly is the "gold standard" of labs (having been around the longest, devised the grading scales that are so commonly used, and having the most established reputation for consistency in grading), but either lab is a solid choice for reliable reports. On the other hand though, AGS reports include some very useful information that is lacking on GIA reports, most notably information on crown/pavilion angles and percentages. This information can be very useful in helping a buyer evaluate a diamond, but GIA reports still don't include this useful stuff.
I prefer AGS. Nowdays GIA is a bit more liberal in its grading, especially with SI grades. On the other hand, AGS is very lenient in polish and symmetry grades. They are both very good, but as I told you, I trust more AGS. Not to mention that an AGS report tells you all the percentages and angles and you don't have to ask for a Sarin.