You don't want them to recognize the car! Where's the fun in everybody driving the same thing!
Don't think about which will be slightly cheaper, just go with the car you like the most. Otherwise no matter how much money you save, after all that hard work you'll just be disappointed with yourself because it's not exactly what you wanted. Why skimp on your dreams when you're so committed to them?
However, consider the fact that the SX will have to be smogged. UGH!
On the other hand, if you indeed want a stop light car, why do an RB at all? Weight is your biggest enemy. Not just the weight of the car, but the rotational mass in the engine. Get an SR and tight gears, up the boost, and be like a bolt of lightning off the line. You're not looking to win a top speed challenge here. For that matter you could get a 510 which the SR would easily fit into and save yet another 200 lbs and laugh all the way to the finish line at those other guys towing a 600lb smog stamp. Of course you'd win by a less insane amount with the RB 260Z but you'd be doing it in style. So then, which gives you more joy, buying car wax or buying tires?