Guitar Amp Pro comparison...

I'll admit that I have never played with Guitar Amp in LE (never played with LE for that matter), but I do know that Guitar Amp Pro is great in own right. I've managed to use it to dial up almost any amp style that I needed. Unless you're planning to play it through a room and micing the room, you will have to use reverb. GAP has a spring reverb (and tremolo) effect on the plug-in that's also not too bad.

jord

For me an importent quality of real guitar amps is dynamic.
A Marshall turned up very loud sounds very bright, when you hit the strings very soft and mellow, but also louder when you pick hard.
This way you can play bright rhythm comping and also fat solo sounds without turning the knobs (or only the ones on the guitar).

The only simulator that has this quality afaik is GAP.

Like many plugins in Logic Express, GuitarAmp use the same algorithms as GuitarAmp Pro but with less parameters available to the user.

That's true but there are some Amp models and Speaker simulations in GAP that are not available in Logic Express.
It's not a question of algorithms.
The characteristics of those are decribed above.

You can get the tone of a J.T.M. 50w plexi (british 50w top) at least and something that's a mixture of a modern Marshall and Soldano (american 100w top)

Since you're specifically asking about Marshall, let me say this for GAP...there's a setting called VH Fair Warning! I thought that showed a lot of guts for even saying they have that tone nailed. But then they add 2 more very distinct Eddie tones to round out the set! And they all sound really great imho. I'm not gonna sit here and say it sounds like a Marshall plexi head, but last time I checked Ebay there were none of those available at any price.

It's hard to judge Guitar Amp sims (to me) cuz I usually end up hating them all after a while, and this one may be no different. But right now it sounds solid and musical and not tinny. It does have a very dynamic wide feel compared to other sims I've tried.

(Sorry to go off on a tangent...I went for Logic Pro just because of the Reverbs and extras, and it was sooooo worth it to me. Best money I've spent in ages.)

As far as Guitar Amp vs Guitar Amp Pro I think Pro is quite a bit better.It has more amp simulations plus the ability to switch speaker cabs, mikes and mike positions. For instance when using Express and Guitar amp I never liked the American Clean sound, it had a weird midrange that didn't work for me.Using the American Clean in Pro and switching speakers cabs I found the a sound more to my liking.

Macbook Pro 1.83 Mac OS X (10.4.8) presonus firebox, logic pro 7.21