Well, just to let people in FT know how this works:

VB (Viva Aerobus): This is the Ryanair of Mexico, and very similar to NK (Spirit) in the USA. The charge for everything and have no selected seats (you need to buy the extra option that tells you the Group you will board with). They do Not let you print boarding passes with cheapest fare unless you pay more. So I would avoid this option if the other two LCC are $20 USD more expensive. You will end up paying those $20 USD in boarding group extra option and in Credti Card charge.

Y4 (Volaris): They are a normal Low Cost Carrier. They have snack and bar service on board for free and you can print your boarding pass and select your seats 72 hours before departure (if flying domestic). Service is similar to 4O (Interjet), being the only difference that they have 174 seats on their A320s vs 150 seats on Interjets A320s. They will also let you check a 15 Kg. bag and carry on a 10 Kg. bag included in the price of your ticket

4O (Interjet): Another normal Low Cost Carrier, and Y4īs principal competitor. They have similar good service as Volaris, as they also have snacks and bar for free and let you check in a 25 Kilos bag. You can print your boarding pass and select your seat before departure and the only extra they have vs. Volaris is the seat pitch, because they only have 150 seats in their A320s.

For me, Volaris is always cheaper than Interjet, so normaly Volaris gets my money on the MEX-CUN/TIJ/SJD routes. Volaris is always priced cheaper than even Viva Aerobus (for when I buy my tickets Apex 90, at least).

All in all, if you want to avoid the Mexican Spirit (Viva Aerobus), then you still have the mexican cuasi Jet Blue (Interjet) and the Mexican EasyJet (Volaris) LOL, which are for me the best comparison of airlines.