Should i buy "used luxury vehicle" or "cheap new vehicle"?

My lease is ending in two month. So, i've been looking around for next vehicle. This time, i'm planning to purchase the vehicle... and here's my dilemma... i'm married with 2yr son... so safety is very important but due to high gas price, i'm giving up suv. I'm pretty serious about buying '06 Infiniti M35X... 20k~30k miles... and i think i can get it under $30K (after tax & fees). When i got inside, i was so impressed (this car made lexus ES & bmw 5's feel like honda civic). Basically my mind was set until one of my friend said... why are you paying almost 30K on a used car? and said that i can buy new accord, camry or altima for that amount...maybe less. I know he has point there... those are nice cars... but i just can't get over infiniti. I believe there are two things i have to try my best not to down grade in life... cars & house. What do you think? am i making a mistake here?