Personally, I love Mexico City so I would fly in the day before and spend a night there to avoid some of the stress and enjoy the city. If you do want to do same day, then the lounge isn't that great and not somewhere you'd want to spend half a day. I'm sure someone who knows the airport better can give specific advice on timings, but the airport can be hectic and it can be a long transfer between the terminals (requires a train which takes you round to the opposite side of the airport - I'd allow at least 20 minutes for a terminal change), so check which one you are flying in to, I believe Aeromexico use both. If you end up cutting down your transit time to just a few hours then there are a lot of flights on that route, so you do at least have plenty of back-up options. Given how cheap the flights can be - often sub-£50, I think I'd look at booking a back-up flight with a different airline at a similar time, just in case of any delays on your planned flight. Although that won't help you much if there are wider issues such as weather or ATC restrictions.