For modelled systems then definitely choose the A2A P-51 with Accusim (both military and civilian variants). If you want a greater variety of models then you really can't do any better that those from Warbirdsim. Your choice will depend on what your priorities are. A lot of people have both. I certainly do.
Outside of FSX another option is to get the DCS P-51. I picked this up in their recent sale for $15.99. The systems are modelled to a similar level of the A2A and the weapons are usable.