I prefer the heavier grates as opposed to the smaller stainless steel
type. The heavier ones hold more heat when they get hot, which helps
put nice sear marks on meat.

As far as BTU's, I've never seen one yet where it was inadequate.
Generally, the BTUs are gonna track the size of the grill. If you have
two that are the same size, but have diff BTUs, the only difference you
are likely to see would be the one with higher BTUs will heat up
faster. After that, I think they will both do about the same job of
cooking. I rarely have mine on high anyway, except when it's first
heating up.

Another new feature is many grills now use a battery for electronic
ignition. Not sure how reliable this is over the long term compared to
the conventional piezo-electric type.

As for propane vs natural gas, I've had both and believe natural gas is
the way to go. More convenient, safer, cheaper to opperate. The only
downside is the initial installation of the gas line. For safety, I
can't recall hearing of an accident with an outdoor natural gas grill.
With propane tanks, I have read of plenty of accidents as well as major
explosions. To refill it, you wind up driving with a tank of highly
flammable gas in your car. Propane tanks can be OK if used correctly,
the risk is not real high and I would use one if I had to. However I
think the claim that natural gas is more dangerous is false..