Just to add to this based on the axles we have done over the last few years and chats with Christo Slee over in the US....
The HDK CVs from Milners seem fine for general day to day driving and overlanding, they struggle when playing on off road sites when the axles are locked up and there are higher impact/torsion loads. I am yet to see one fail on an Overlanding vehicle.
I suspect that the factory CVs will fair better in these situations, but whether at five times the price they are five times better I don't know.
Longfields are designed for that sort of thing so if pay and play sites are your thing, start saving the pennies, however they do wear quicker so probably not the best thing to use for day to day driving or overlanding.
As for Milner seals, unfortunately the only buying policy they follow is price, getting things as cheaply as possible, so you may buy a complete seal kit one day with half decent seals and then order another the next day and some of the seals my be missing and the source and quality of others will vary considerably. The bottom line is be prepared to be fussy and send the seal kit back if you aren't happy, otherwise if you can afford it stick with factory seals.
The same goes for bearings, one day they could be selling Koyo and then next some cheap copies. The day I bought a set of swivel bearings and ended up with a mixed set is the day we started to stock our own genuine Koyo direct from the main distributors.
As for the Trail Gear inner axles seals they do last longer particular on trucks used on pay and play sites or US style trails where there is more movement in the driveline, however they prevent the leaks by gripping the halfshaft more firmly so not recommended for long term use because they wear groves in the halfshafts.
At the end of the day the best investment is to stick to Toyota seals, it's not worth messing around.