I guess I'm not invited to this thread because I don't meet the requirements :(
I've never owned my own installation shop, and I didn't come close to making $150K when I was installing at Circuit City to puy myself through college (earning my 4 years electronics degree). However, I do feel like I can provide some insight because now, after many years of installing, I have designed and provide technical support for >10 mass marketed consumer electronics products specifically designed for the harsh automobile environment (dirt, dust, moisture, vibration, temperature). This is through a private LLC that I founded and am still the majority partner in. We also provide a 'no questions asked' limited lifetime warranty on everything we sell, so we MUST stay on top of quality.
After college, I earned a national reputation as a controls engineer contracting for companies such as BP, Marathon Oil, Sonoco, Chevron, Citgo, etc. I've wired and rewired hazardous location machines all over the USA. I am currently employed by one of the largest packaging companies in the world and work as an automation/electronics specialist..
Oh, and to top it off, I just got done installing 3 3M Scotchloks, so I have first hand experience with them.
Providing technical support for car electronics can be a very frustrating job, especially because 99.9% of the time you are not on site. So, my first question I ALWAYS ask a customer is "How did you make your connections?" If they say scotchloks, ttaps, etc. I always instruct them to have the wires soldered, or at least check the wires for continuity. This typically fixes 60% of problems we deal with (rough estimate, we don't actually keep track). To be honest, the majority of the problems stem from using a Ttap with two different gauge wires - a VERY bad idea.
From an electronic standpoint, I seriously doubt you will find any noticeable difference between any fresh connection. Over time, and I've seen it in the field, barrel connectors and other metal connectors will corrode and start causing intermittent problems. I've made more then one middle of the night service call because of blue barrel connectors!
From a reliability standpoint, soldering is the way to go, hands down. It is the only permanent splicing method. All other forms of splicing relay on MECHANICAL forces to keep the copper together. As stated in the other thread, vehicles are rattle traps, meaning they are metal boxes built around a combustion engine - every thing vibrates inside them, including your Scotchloks. Over time, they will start to back out, just like a bolt or a nut would (locktite is an amazing product). When soldering, you bond the wires together. Short of the wire actually breaking, there is NO chance it will EVER come off without reheating the joint to >650 degrees F. A quality tape MUST be used to protect the splice (such as 3M - it will NOT peel). 3M has been the only tape I've ever used, from CC through the oil industry, and now.
As stated above, I just installed 3 #M Scotchloks (model UY2). We started using these a few months ago for small control wires that were to hard to solder with the factory equipment we have (22-26 awg). This was done for SPEED reasons, because when a production line is down, we are required to fix it immediately.
Anyway, the UY2 states right on the box that it is 'moisture-resistant'. The connector actually contains a sealing liquid on the inside. Once you crimp it down, the liquid fills the splice area and oozes out the opening. I don't think I would have any hesitations using these in a car, but without the goo and without the 'moisture-resistant' designation I wouldn't even consider it for a long term install.
I just searched 3Ms site and the TTaps you are referring to are NOT designated as 'moisture-resistant', but are recommended by 3M for automobile use.
I personally think it is funny that you claim it takes 'many' cars to learn how to use these things. What do you do for your first 5-10 customers while you are 'learning' how to splice wires? That remark right there is reason enough these things should never be used in cars!
At the end of the day, though, just keep in mind that every time a customer takes one of your installs to a shop that solders that shop is going to make you look like an amateur. It is VERY easy to demonstrate while YOUR customer should pay ME to remove all the 'crimps' and 'melt' the wires together. When I worked at CC while in college our shop was literally right next door to a custom shop. Every January I would redo at least 10 remote starts and remove all their scotchloks. The customers would gladly pay for it and remark how they would NEVER go back to that shop again because of the shoddy work they performed. The customers would come to us because the original shop wouldn't always tell them it would be 2-3 weeks before they could look at the car. Natually, with us being right next door they would drive right over and get our oppinion. Using a Scotchlok I kept in my box I would show them the difference between the two connection methods. We made LOTS of money!
Kevin Pierson