There are very few circuit differences in the main board of both amps. The EQ board is where they go their separate ways. That and the TSL has a few extras thrown in such as the VPR mode (which simply limits the signal coming off the phase inverter plates, if i recall correctly), and some FX loop extras. Both suffered from some manufacturing defects along the years, in the form of bad parts and poorly made circuit boards (issue 5 comes to mind) but overall a properly cared-for JCM 2000 can be very reliable.
Tonally, they're both workable in a pinch as stock, but i have a set of base mods i perform on every JCM 2000 that comes across my bench that helps unlock what they are capable of. My modded DSL totally slayed my H&K Triamp in a medium-high gain shootout.