500I gets my vote. Buy once, cry once. Like Hermio wrote, use it, take care of it, and if you decide to part with it....
uh... IDK if the source you're citing is the most compelling (OP: unless you've been lurking for a while and caught the thread on 'sawdust in the sparkplug hole killed my saw' then you'd have no way of knowing a certain not-to-be-named-by-me poster got duped by a dealer and ended up with a 500i)
That said the 500i is admittedly a VERY compelling saw... the fuel injection is making horsepower numbers that weren't possible from a saw with the same displacement in years past... and in a much lighter package...
As for me, if I'm looking for big power I'd rather get it from displacement. In ~5 more years I'll trust the "new" fuel injection they're running on that saw (its not that new but I'm a grumpy old man)
but... (shrug) I suppose the lesson cited is another guy who ended up getting a saw he neither needed nor wanted... he bought it "because it was such a good deal"
As for YOU, OP... you got your eye on that 400? I've been eyeing it, too. If someone says the words "magnesium piston" I stand at attention. I suspect if one of those Stihl calendar ladies said it I'd get instantly hard LOL
You know what you want--go get that 400 with those FEMA dollars. Rebuild/repair your old saw because frankly, its a badazz saw, too and if it were me it would be a keeper.
Worst case you can use it when you send your new 400 out to get ported
I think the 400 is going to be an all-time classic because the "upgraded" model WON'T have the magnesium piston... plus its a different bore so people won't even be able to retrofit a magnesium piston into the new models...
Personally, every motor I've ever had (whether it was a chainsaw, motorcycle, car... whatever...) the before/after difference of getting a lightweight piston in it is NIGHT and DAY... less rotating mass causes the motor to spin up faster, smoother and once you've experienced it you're kind of ruined for life because unbalanced motors with heavy innards just don't feel the same anymore (don't get me wrong, diesel brothers... I still love the low revving slow burn of the diesel chug... enough torque to tow a house)
But if I were in your position I's already have that 400 in hand and I'd be posting about how in love with it I was... and I'd be saying "I'll be ruined, you say? RUIN ME! RUIN ME! RUIN ME!"