Well, an object may be prettier usually, but in PHP I usually feel more confortable with arrays (maps actually). If I had a PHP editor which had an auto member listing, then it would be another story, but as I don't know about such editor I feel more confortable with arrays... cause I can do things like print "lalal $array[keystring]"; instead of print "lalal {$object->member}".
Anyway, as mar77a said, it's not really a lot of ram, but it's good that you care about clearing up useless stuff.