The second, because the first will throw a PHP E_NOTICE error if that property is missing.
You could also do this, but getOption() is a lot cleaner and clearer:
if (isset($scriptProperties['propertyName'])) {
$prop = $scriptProperties['propertyName'];
} else {
$prop = 'some default value';
}
getOption() also gives you more flexibility with its last two arguments:
$prop = $modx->getOption('propertyName', $scriptProperties, 'default value', true);
The default value will be used if the property is not set anywhere (tag, default properties, property set). This bit me hard long ago because if the property exists in the default property set, isset() will return true, even if the property is empty and has never been given a value.
The fourth argument, if true, tells getOption() to use the default value if the property is set but empty. That's handy if the property exists in the snippet's default properties (or anywhere else) but is empty. I use it often, unless 0 or an empty string is a possible valid value.
The third argument is null if you omit it. The fourth argument is false if you omit it.
FYI, it's not likely, but if the property is not set, getOption() will check SystemSettings, User Settings, Context Settings, and User Group Settings for the key and return the value if it finds a match. Moral: make sure your property names are not the same as any Setting names (e.g., emailSubject). Using a prefix on your properties will make sure that doesn't happen.
