Opinions are divided on this...
Many say crimping is better because it allows more wire flex before the wire breaks off. This is definitely true, but obviously if the wire is really allowed to flex it will break off anyway, so this just buys you more time.
Crimping is also better for less than competent installers because the crimp tool is pretty well foolproof unless the operator is completely blind.
The tools are very expensive - e.g. the M22520/2-01 sells for between €400 and €800 depending on the make and what you get in the kit. This one is £555. To my astonishment I have just e-sniped an ancient but apparently good condition DMC one on Ebay for £60... clearly nobody else noticed the auction.
The crimp connectors are also eye-wateringly expensive, especially the good quality Positronic ones. Some of the pins are £2 each, so multiply that by say 25 ...
Soldering requires more skill but the joint should be much more immune to corrosion, and even the most expensive heavily gold plated connectors are much cheaper than good quality crimp connectors.
I expect that these things vanish in the traditional £2000-4000 avionics installation fees but I wonder if anyone has any views on these options. I tend to pre-make harnesses on the bench which saves me a great deal of money on avionics jobs, and I solder everything.
A matter of personal taste and age. Older avionics people tend to solder, younger tend to crimp. Each will tell you why their method is far superior. During my last large avionics upgrade, both methods were used on my airplane and I got the two guys started arguing which method is better. I only had to listen and note down all the arguments
The other thing with soldering is that you really should heatshrink over the pins - both to guard against any wire whiskers, and to give some strain relief. This takes time... but if you are prewiring something yourself, time is not an issue.
My neighbour and good friend is an electronics design engineer and insists that crimping is better than soldering for harnesses because the act of soldering immediately starts the corrosion process and creates a weak flex point at the same point as where the corrosion will start. FWIW.
Soldering itself starting corrosion is news to me. Only if you are using an aggressive type of flux.
It may be that a corrosive environment, over many years (e.g. a plane parked outdoors) will affect soldered joints more than crimped ones.
But if the soldered joints are onto solid metal pins (not pins formed from sheet metal, which is how cheap connectors are made) and then heatshrink protected, the solder joint should be protected fairly well.
One issue is that if trying to connect to 44-pin D connectors such as these nobody seems to make connectors with solid pins. All 44-pin connectors I have found have the cheap and nasty formed pins. The only way to get solid pins is to use crimp connectors, and you have to pay a lot of money to get quality.
My 0,02 euro: crimping is superior to soldering, theoretically; but the difference isn't spectacular, and a well done solder joint can serve for many years. Far more relevant is the use of quality components and tools - especially when crimping, and that's not cheap (all of this already been said, I realise) . Most important is to guide and fix cables and cable bundles correctly, to keep stress away from joints and connectors. Also, when laying out cable, think of where the unavoidable water and dirt will go. They do have to go, so create a way for them, and no places to accumulate.
I was told solder joints are affected by vibration. (I used to solder quite often -but not in a vibration environment)
I was also taught to crimp rather than solder, but I tend to do both. I just wired up a new comm and intercom harness. The connections I removed were crimped and poor.
For most avionics connection pins, there is a electrical connection, and separate strain relief crimp. When it is possible, I crimp the electrical portion, and then solder the end of wire end of it. Then, I crimp the strain relief portion of the connector. The intent being that the solder has not wicked down the wire that far. The strain relief crimp can then do it's job, with wire which does not have a weakening point soldered into it. So far, so good - and the radio works!
Speaking of expensive crimping tools, some prices are outrageous, but apparently there are also good inexpensive tools. During last year's annual, I asked the avionics engineer (whose main job is at BA, where he has worked for 30+ years) what he considers decent quality tools for avionics work, and he happily referred me to LAS. Their crimpers are priced from £11 to £100.
Speaking of soldering vs. crimping, he said that crimping, if done properly, produces fewer bad joints than soldering, and that a proper crimp is actually a contact weld. Also, when you need to connect hundreds of wires per day, crimping is a lot faster.
The intent being that the solder has not wicked down the wire that far.
There is a definite issue with that; the wire (under flexing) breaks off right at the point to which the solder has wicked, and this is why soldered joints must be protected from flexing.
However I have never come across a soldered wire which broke off and which was anywhere near suitably protected from flexing, e.g. by heatshrink sleeving over the joint and the wire bundle coming out via a hole in the connector backshell.
there are also good inexpensive tools
I know LAS fairly well and there are some very cheap crimp tools, but I think the tools for the classy machined pins are always pricey. Here is a (large file) photo I took with my phone via a 40x microscope at work, showing a Positronic female pin for a DB25 connector. Those pins are outrageously priced at something like £2 each. I used these to make an extension cable for my EDM700 sensor connections (13 thermocouples) and on those soldering was not an option anyway (chromel/alumel) short of using corrosive solders which need to be washed off, or silver-soldering which would melt everything else around. And the only crimp tools I know of which do that specific crimp, widely used in avionics, are expensive.
For example I think these
will always need fancy tools, whereas these
can probably be done for peanuts (and will last about as long as Mr Ratner's prawn sandwich).