The thing is Mech Maniac is more skill dependant than Gun Maniacs, because yes Gun Maniac got huge dps but that's only under cast time, and they also need to aim for the head to get the bonus 50% damages, for exemple Desperado's shoot has a 2 second cast time and does 847% physical damages, but it misses if it doesn't hit the head of the target, so it's all situational, a bit like fighters, that's why for people who would like to play Gun Maniac instead of Mech's i would recommend you not to lvl Desperado because it's only based on luck, of course you can buy the item thats increases the chances of hitting head's by 5% for 1290 cash points, but i don't think it's worth it, anyway you should level Starshoot aswell as Bull Shot, these are the most reliant spells of the Gun Maniac, because they both have over 500% dmgs (507% Starshoot, 554% Bull Shot) and has 50% bonus damage if crit and are NOT based on luck, but the counterpart to this high damage output is that whenever they use one of these spells, they get 1 point of Heat status and it decreases their defenses and resistance by 10%, so having a maximum of -50% defenses + resistance is risky, you gotta make sure you're wearing full DEX + crit % set aswell as a crit% headgear to make full use of these, but again, Mech Maniac has Flying Bullets which are flying all around your character increasing your defenses by 30%, so since they have the master class passive that gives at lvl 6 a total of 2927 LP and 10% reduction of all damages, they can survive with ease Gun Maniac's burst with a Physical Defense set full +12~15, ending up outdamaging them, and i ain't even talking about the Rapid Shot that has a 1 second stun at lvl 2 (if maxed lvl 6 it become 2 seconds but not worth it IMO) for only 5 second cooldown, so you can interrup the long cast time of Gun Maniac's anyway, this is the reason why Mech Maniacs are so strong tbh, but of course you will need to spam these EP pots lool.
Also it seems like you guys forgot about the Combo Spell of the Mech Maniacs loool, when you summon your Flying Bullets followed by Rapid Shot + Deadly Headshot, you unlock the use of Snipe which does 5000 damages that cannot be reduced, of course it will put a global 10 second cooldown on all spells after this combo, but if you proc the Flying Bullet + Deadly Headshot combo, it will send all Flying Bullets to the opponent, dealing 80% energy damage per bullets (you will summon 5 bullets that will grant you 6% bonus defenses each, losing 1 per attack received), on top of the 416+293% physical damage of Deadly Headshot, so the burst will be giant and you will probably be able to end a fight if you use it for a finisher, the best way to use it is to use Kaioken right before the combo lol, yesterday i killed Zophar with this combo, here's a screenshot :
edit: alright so according to your posts there is a shared opinion between Gun & Mech maniacs, so i'd like to show you this Poll created by Akira Toriyama himself (here's a pic proving it

) LINK :
http://strawpoll.me/531547/
Anyway yeah go ahead and vote for who you think is the best class !!