I am looking at older Webers and am a little confused with the differences between models. In the local ads, there is a Weber Platinum series grill for $150, but it was natural gas and it appears that the cost to convert to propane is more than it would be worth rather than to just be patient for a propane model. I've seen other models that say Genesis, Summit etc and the design appears fairly similar other than a few minor differences. Can someone please explain one model may be more desirable than others and why?