fusion engines will only brake due to operator error, polarstar have done tests where they hook it up to a compressor and ziptie the trigger down for more then 2 million rounds. as for amount of rounds being able to be shot, if you have it set up correctly and a average sized barrel you can get 4000-5000 rounds out of a 90/45 tank. if you plan on shooting a lot what we have is a 110cbf steel tank that we use to fill our tanks for milsim games.
Not entirely true, but for the most part they are pretty bulletproof. I have, 6 polarstar guns at last count, and about a dozen AEG's ranging from $150-2500 per gun.
Do you think a custom 1000$ DMR (Todays parts) will last and preform well for the next 5 years? Or will airsoft guns/upgrade parts all be better by today's by then?
You can make an AEG last for a long time, if you are willing to spend that kind of money. There are always innovations coming out to make airsoft better, but in general nothing revolutionary has come out to make them hugely more durable other than indie mods like sorbo/aoe/radius windows/etc.
They both have their pros and cons.
Polarstars, for me, sound awesome. They sound like a 22 or even something bigger if you overvolume them a lot. There is nothing that says 'airsoft' about it when it comes to its sound. Sure, there are a lot of other pros to the engine, but the sound is what sold me on it. The tank on your back, you don't even notice, nor do you notice the hose of a polarstar, even if you are running sling-less, I've never noticed the hose while in a firefight.
Aegs, they have the pro of using a power source that fits inside the gun. An an overall system, this generally makes them weigh a few lbs less and use less real estate on your back. For most airsoft events, this is a non-issue since you aren't hauling multiple days worth of gear/food/ammo/etc with you in your pack. For multiday ops like East Wind, this becomes an encumbrance.
Both can be made to be similar in accuracy if you have a specific weight BB and FPS you are tuning for. Polarstars have the benefit of being able to adjust for both weight and FPS much more easily than most AEGs, but this is not universally true.
End of the day, try out both, and see which you prefer. To reiterate, its the sound that sold me on P*s. You can tune aeg's to do what you want out of them, but you can't replicate the sound (with one exception, of a device that does not exist yet in the USA.. and it adds considerable weight).