f430 - I'm in the exact same predicament that you are... deciding between two wonderful cars. I have a couple of friends with Gallardos - one with E-Gear and the other with a stick. The Gallardo is a very nice looking, high performing, dare I say reliable, performance car. It is all wheel drive and that makes it very forgiving when you drive it quickly. Ingress/Egress is very good and the car is VERY easy to drive. There are a few people over on the Lambo forums that use them as everyday cars. Thanks to Audi's influence, they have become very reliable if not a little too "common" on the interior design elements. Maintenance schedule and associated costs are very reasonable when compared to an F-car. From a cost perspective, the G and GT cars are similar. You can find a low-mileage, prestine Gallardo in almost any color you choose for less than you can find a GT for. Of course you can go newer and spend more than you would on the GT.
Regarding the GT.... I just can't get over the lines of the car. As good as the GT looks in pictures (and they look damn good!), the GT is even more attractive in person. After all these years.... the shape is just timeless. The Gallardo is space-ship looking good but the GT is, in my book, very masculine - much more so than the Lambo. I think I could sit out in the garage and look at the Gallardo for days and weeks - but the GT for months and years.
Getting into and out of the GT is a pain in the butt compared to the G. The GT would not be as good for an everyday driver.
The GT is a "raw-er", "edge-ier" sports car compared to the G. Of course, the GT has all of that raw, raw, Americon icon heritage which does weigh (positively) on me too.
OK, all that said, a very close friend of ours who has known me for the last 20 years kind of summed it up best. She said that if you want people to know you've arrived and "be noticed" - the Gallardo is a good choice. It's a great looking exotic car that can back up the show with "go". The GT, on the other hand, is purely a car lovers car. I can see non-car people with the means ($) driving Gallardos. However, I think the majority of the GT drivers will be genuine car guys - first and foremost.
Just so you know my bias, I'm probably ordering my GT tomorrow....
(My $.02... feel free to agree or disagree.)