I would simply buy the XBox 360 game anyway. I would not listen to what the GS employee was saying. Quite frankly I do not care about what people have to say. Gee it would not impact on my day at all.
Anyway if the GS employees are trying to stop customers from buying 360 games which often sell more copies than the PS3- their days working at GS would be numbered.
Retail employees have the right to sell stuff- not stop people from buying things. No one cares what the sales reps have to say.