homebrewer is 100% correct. you don't want tall stretchy plants in veg. he's wrong though cause it has nothing to do with the difference between the nitrogen in the 2 formulas.
As usual there's elements of truth and fiction in everything.
Homebrewer is correct that the type of nitrogen does affect the plants. He's incorrect in saying that Advanced Nutrients uses only one kind of nitrogen.
Ammonium nitrogen produces greater petiole leaf expansion. Nitrate nitrogen, in too large a quantity, will cause shorter petioles, smaller leaves, and thinner stems. So the key is to blend the two which is what any good fertilizer company does. However, a lot of them are just like the Label Lawyers out there who look at the sticker on the back of the bottle and decide that because it lists the same numbers it's the same thing as another bottle. I don't care what you're talking about, the ingredient list on labels are crap. The rules about what can, can't, and must be on a label are flat out retarded. They completely prevent any company from actually accurately listing what is in their products because a lot of them don't want to in the first place.
Look at the label on the back of a bottle of grape juice. Then look at the label on a bottle of Coke. They look pretty similar. But one is good for you and the other isn't. Why? There's tons of stuff in grape juice (flavanoids and so forth) that are absolutely awesome nutritionally but aren't on labels because the bureaucracy hasn't decided yet whether or not those things are "officially" good for you.
Plant nutrients are even worse because for the most part, the government cares a hell of a lot less what company's sell to feed to plants compared to what they sell to feed to people, despite the obvious fact that people eat plants grown on those fertilizers (or smoke them). Fertilizer labeling laws are pretty much all based off tech that dates back to the 70's. Botany has come a LONG way since then, but you wouldn't know it by reading a label.
So you get the companies out there who look at the competition's label and think "aha! We can use this to copy their formula!" and they go and make a fertilizer that has the same NPK. Whoop-de-freakin-do. You could quite literally make a "fertilizer" with the same label ingredients as a major brand that is COMPLETELY useless to plants. There are ways of getting the same amounts of each element into solution that are so bio-unavailable as to be practically worthless to a plant. You can pee in a reservoir and massively increase the nitrogen levels but unless you have the microbes that turn the nitrogen in urea into a form useful to plants you're not helping them one bit. Labels are good for getting some idea of what's in a bottle, but they BY NO MEANS actually tell the full story.
Back to the original point, it's not a matter of one thing being good and another being bad when it comes to nitrogen. It's that there's more than one good thing and - as with most things - it's a matter of how much and in what ratios. Too much of a good thing is just as bad as a bad thing.
So the type(s) and amount of nitrogen does influence how the plant grows. Virtually every macro and micro nutrient has some impact on how the plant grows and the source of those nutrients also make a difference.
Oh, and here's a fun tid-bit. It takes full-on laboratory analysis to truly reverse-engineer a fertilizer. What's more, it's possible to use masking agents to make that analysis difficult if not impossible. I have heard that Advanced Nutrients does that to protect their formulas but that could be just a rumor, I don't know. But it makes me a lot more skeptical of anyone that says they've copied AN's formulas.