Alright, so Dragneel has already given an excellent answer but I'm going to put my opinion here and I'd back Bronzong over Spiritomb in Gen 5 (or under) and 6.
From a Gen 5 view;
While it is true Spiritomb has no weaknesses it only has 1 resist and 3 immunites
Bronzong on the other hand, has an impressive 8 resistances and 2 immunites, as well as sporting higher defense and special defense at base 116, to Spiritomb's base 108, and a base 67 HP which outclasses Spiritomb's base 50 HP.
Offensively speaking, their Attack stats are very similar with Bronzong being base 89 and Spiritomb being base 92. Spiritomb's special attack outclasses Bronzong (Spiriomb - 92, Bronzong - 79).
Spiritomb has no weaknesses and Bronzong has 2 - fire and ground. However, courtesy of Bronzong's abilities, Heatproof and Levitate it can avoid one of those, giving it 1 weakness - and it can potential avoid the other by running Bronzong with Levitate and in rain, therefore nullifying both it's weaknesses, and leaving it with none, just like Spiritomb.
Movepool wise, if you look at Spiritomb a large part of it's movepool consists of Normal, Dark and Ghost type attacks. Bronzong's offensive movepool isn't that great either, but it does get access to moves like Gyro Ball which lets it abuse it's slow speed and Earthquake, which is a great coverage move.
From a Gen 6 View;
Okay so in Gen 6, Bronzong went up to 4 weaknesses, with Ground, Fire, Dark and Ghost - though it can still avoid Ground and Fire courtesy of abilities/rain. Spiritomb gains the Fairy weakness. I would still however, back Bronzong, courtsey of his new resist of Fairy, and his better defensive capabilities and still acceptable offensive qualities making him a good tank.
Anyway so yeah, I'd use Bronzong over Spiritomb. There's also a reason that in competitive battling, Bronzong is in the UU tier with the ability to be used in OU and Ubers, while Spiritomb is in the lower RU tier. Of course, it is still your choice and it also depends on your current team.