Too slow falling edge
As-is, your first problem is that your circuit will draw crazy base currents from the BC369 PNP guy, as well as the 2N6488s.  The 2N6488s certainly need a lot of base current here (100s of milliamps), but it shouldn't be as "unlimited" as your current circuit allows it to be.  That's probably breaking the simulator's models, as it implies a tremendous amount of saturation charge which has to be cleared before the transistor can switch back off.

Here's your circuit in CircuitLab:



I've added a few resistors (R3, R4) to help control the currents a bit.  Click through to the circuit page, run the DC and time-domain simulation, and you'll see closer to tens-of-microseconds switching times -- not the 500us you're seeing.  Hope that helps!

EDIT: R4 isn't super important because there's already "feedback" from the rising base voltage.  But R3 really is, because it sits between Q1 and Q2, both of which have their emitters tied to opposing rails, so they don't have that same feedback mechanism.