But is LTSpice not restricted to designs around LT's parts ?
No, not at all. It understands basic SPICE syntax for device models and has appropriate extensions for behavioral modelling.
Most casual users will miss the convenient device library stock shipped with PSpice. But it's no problem to import vendor device models to LTSpice.
Some more advanced PSPice features missing in LTSpice (also not available in the basic PSpice version, by the way) are:
- mixed signal modelling with digital circuits
- circuit optimizer
- Monte Carlo analysis of parameter tolerances
- post processing flexibility
I notice, that LTSpice has been choosen as an every days simulation tool by many professional developers, in favour to PSpice.
P.S.: You'll find simulation cases, where one of both tools fails, mostly in transient analysis convergence, and hold the case against the tool. The issues refer to either inherent weaknesses of the basic SPICE algorithms or specific implementation parameters. I can't decide at first sight which one is "better" in this regard.