An Engine Management system is a crucial upgrade, particularly for the mods listed in this thread.
As far as "which" EM system is best, that is entirely personal opinion. I'm sure the low-cost EM (tatrix, etc) are just as good as the Cobb AP, provided you KNOW WHAT YOU ARE DOING. I've heard horror stories of people opting to buy "self-tuning" software that have no idea what they are doing, and end up blowing something up.
I'm FAR from a computer expert, and quite frankly, when I start thinking about the science of air/fuel ratios, etc, I get frustrated.
I don't think anyone can argue that the COBB AP is by far the most "user friendly", particularly for newbs or computer idiots like myself. I also don't think anyone can argue that Cobb has outstanding customer service.
I'm quite certain that MANY will argue with what cobb charges for the Accessport though
Hopefully, the buyer is well educated on the differences, weighs the pros and cons of each, and smart enough to know that he/she needs some kind of EM before bolting on that brand new GT40--LOL