The differences of nutrients for different crops are predominantly a case of concentration depending on the growth state, with fruitbearing plants (tomatoes!) requiring additional nitrogen and calcium when the fruits come out.

Your three nutrients have the following NPK Ratios:
Grow: 2-1-6
Micro: 5-0-1
Bloom: 0-5-4
(You can work out that approximately:
Grow = 14% Potassium nitrate + 2% Potassium dihydrophosphate
Micro = 27% Calcium nitrate + 2% Potassium nitrate,
Bloom = 10% Potassium dihydrophosphate)

This information will allow you to mix 'n match your nutrients for each crop.
Please, also check out: pH/PPM/CF for Various Vegetables

That's a great link.

Does anyone know what the difference is between the two brands?

I don't know any specifics of what exactly is different between GH and Advanced Nutrients' 3-part nutrients, but from what I've read and seen the AN stuff is better.

I suppose it depends on how important it is to you for your plants to be as healthy as they can be.

I've always used GH three part solution with great results.
I'm gonna try chucks Southern Ag hydro formula starting this week. I hope it works out as the GH stuff is expensive on anything but the smallest of systems.

Advanced Nutrients 3-part is the best. I can go into a big long explanation if you like, but it's the better product.

Here is a link that might be useful: Grow, Micro, and Bloom

I went ahead and got the AN three part. Seems great so far, I'm seeing a lot less big swings in pH and the plants are real happy.

I'll let you know how it goes.

Quick update:

Not a whole lot to report. Everything is super-healthy and almost violently green, but I'm still a ways from getting any kind of harvest. My pH has been so stable the last couple weeks I'm tired of checking it. I don't even need to up or down it to start with - just water and nutrients comes out good.

So far it's the Staples Easy Button for hydroponics.

Forgot to update this one.

I just brought in that crop of greens and started another about 10 days ago. This couldn't have gone smoother and the plants I grew were huge.

Good stuff.

Well I ended that crop and started another with the Advanced Nutrients. It's everything they say it is. Two thumbs up from me.