The TBI units have the same problems as carb when it comes to fuel distribution between the cylinders. Multi-port injection addresses that issue by having an injector for each cylinder.
The big advantages to TBI systems are (in nor particular order):
-You don't have to change the tuning for environmental effects such as temperature, altitude, etc because the computer manages all of that for you.
-You don't have to worry about floats, needles, etc. Fuel pressure has a much lower effect on a fuel injection system.
-The fuel system is sealed, so you can leave it sitting for months and it will still start right back up without cranking and pumping and fighting with it.
-Closed loop tuning is possible using an O2 sensor.
The GM tbi units are pretty robust. There are a decent number of free tuning tools available for them, but a prom writer is required. There is also a company (I can't think of the name at the moment, will add if I come up with it) that sells tuned chips for different engines. Parts are easy to come up and fairly cheap.
The FiTech units (in my experience, Coilover seems to like them better) tend to have a much larger issue with inconsistent fueling per cylinder. They claim that their annular ring injector head design helps with it, but their design is kind of [censored] poor. The pressure differential around the ring results in more fuel spraying in from the injector sides. With it coming in sideways, it does not distribute well. If you look at the promo pictures they publish, you can see the uneven fuel distribution. If your intake works with the fuel distribution of the injectors, the autotuning works quite well.
The Holley system has a much more traditional design based on the GM sysem. You can actually use the GM TBI unit with the Holley controls if you are willing to get creative with sensor installation. The tuning has more options than the FiTech units.
Both the FiTech and Holley units can both control your timing, which is a nice bonus. I know the later (late 90s) GM systems had a smart distributor that was controlled by the ECU, but last time I tinkered with on you could only use that feature with a lot of extra hassle.
I have not personally used a megasquirt, but I have looked at them several times and been unimpressed. You have to be careful to get one assembled by someone competent. There are a lot of sketchy sellers that don't use quality parts or assembly practices. They have a limited codespace and tuning options (number/size throttle tables and such). If someone is considering one, I strongly recommend doing some in depth research.
Going with an aftermarket ECU like a Haltech is not cheap, but you can use it with a great many hardware options and the tuning is extremely flexible. It has a lot of options for boost, nitrous, transmission control, etc. I am toying with getting one for my daughter's j**p project.
Might be more info than you really wanted...
