Generally, I'd say if it's on a 737 or RJ (as your flights will be), then it's not really worth it for those recliner seats. However, since this is an overnight flight, then I'd be tempted to take it, just so it would be easier to sleep for a few hours.

FYI, if you don't upgrade, I would splurge $30 for a daypass to their Salon Premier lounge at MEX. If you have a couple hours between flights, I think it's worth it.