Implosion is, IMHO, less likely.
For it to implode, the pressure hull would have to fail at a depth significantly shallower than it's test and previous dives. Sure, that could happen, but the hull itself is actually pretty well designed and 5" of carbon composite is strong enough to hold the pressure barring damage.
I'm assuming what caused the sinking also caused the commo loss (Occam's razor and all that), so whatever happened happened 1.5 to 1.75 hours into the dive, which shoulf have been only(?!?) 6000 or so feet deep.
On the other hand, is she sprung a leak, either a failed hatch seal (or improperly sealed hatch) or the aforementioned not quite up to snuff porthole, then the water would rapidly start filling up the hull, decreasing buoyancy and speeding up the descent. Things would rapidly accelerate as the sub got heavier, and more air was replaced by water, but by the time she got to hull crush depth the inside would be full of water, and the pressures equalized, so no implosion. If that porthole gave way, all that would have happened in seconds.
So that's my money: She sprung an unsurvivable leak that filled the inside with water and she sank from about halfway into the dive. She's probably sitting half buried in the muck she augured into full of water and 5 bodies. But that's just me betting on the odds.