The 964 is a sweet car... Given the option of the two 996 vs 964 it would be a total personal preference of what you like... they both look great and as long as you keep up the maintenance you should be fine with both (even in hot weather - although the liquid cooled engine is a peace of mind in the heat)... having said that the downside is that you have to maintain coolant system (fluids, hoses, pump etc).
From a driving perspective the two are totally different... the 996 (as you said) is more like your boxster.... more refined, the 964 to me gives you a very connected feeling even though its not as refined as the 996. The classic look of the 964 vs the newer 996 look - all in what you prefer...
Have a look at this youtube video.... :-)
Porsche 964 vs 996 - Total 911 - YouTube