Toyota and Nissan are pretty close when it comes to reliability. I still trust Nissan more. My mom and dad both have driven Nissan/Infinitis for the past 20+ years and I've seen many of them last over 200k miles.
My wife's family on the other hand swears by Toyotas and all three of their's leak oil and have other issues.
As far as which to buy? It depends on what you want.
Sports coupe or luxury sedan? The G37 is pure performance. It's gorgeous (minus the ugly chrome lip) and will whip on both the IS350 and BMW 3 series.
The IS250 is very luxurious (at a price), but is lacking in the performance arena. You'll probably get much better fuel economy out of the Lexus due to the smaller/less powerful engine though.
As for AWD. Who cares?! If you drive in the snow, you don't get a super fast, super powerful RWD sports coupe...You get a G35XS=Power, luxury, sportiness, AND AWD ; )