In-game
If you are playing HG it really depends on the starter you want since Chikorita(Meganium) and Bulbasaur(Venusaur) are both really good in their own way. However you will find yourself focusing on hitting harder then being a defensive staller in-game. For this very reason I would say pick Bulbasaur as Venusaur will be doing much more damage then Meganium in-game. They share a similar stat distribution however Venusaur is specialized in attacking and it sports a good Special Attack, Special Defense and decent speed. With these stats it will take hits well whiles being able to hit back hard with it's powerful moves.
However Meganium is certainly not a bad Pokemon to use in-game and is no single aspect weaker then Venusaur. This is because of it's bulk which will often make you tank hits that would otherwise form a threat in-game. Though it doesn't have a dzzling special attack, it is decent and is high enough to be doing some damage. It is equally fast as Venusaur and sports better defensive stats.
If I were to say that one of them is better then the other, then I would be lying. However If you want to hit hard and not stick in a battle for long, then I would advice Venusaur as it will be hitting much harder.
If you like to enjoy the battles and make them a little more interesting, I would say pick Meganium because of it's high bulk and decent offensive abilities, battles will usually take a little longer then if you were to use Venusuaur since Meganium is not particularly an attacker, more a tank
Competitive
Competitively this is exactly the same. None of them is better then the other, however Venusaur is in a higher tier then Meganium. This isn't because it is "better", this is maily because it got the hidden ability Chlorophyll. This doubles it's speed in Sun and when combining growth with this strategy and if it has good team mates, Venusuar becomes very deadly.
However Meganium wasn't blessed with an offensive ability, rather a defensive ability. Though it has very nice defensive stats, many Pokemon in OU are to powerful for it to tank on. For this reason it has fallen to NU. However it does it's job fine in NU. It's bulk isn't rivaled by much and because it has a huge amount and access to status inducing moves such as toxic, Meganium can be a pain to deal with
Conclusion
in-game
In-game they are both equal however if you want an attacker go for Venusaur
If you seek a defensive Pokemon, go for Meganium
Competitively
Although I don't enjoy saying this, competitively, Venusaur is better since it can profit from it's DW ability, Chlorophyll. This will help you do some serious damage after just 2 growths or even 1 in sun. Venusaur also resides in the most common tier which is known as OU so you will often find yourself finding it more useful then Meganium in that tier
Though residing in the depts of NU, Meganium is a nice Pokemon to use if you favor the NU tier, however you must take note that there are other Pokemon that can do the same job as Meganium but only better
Competitive wise, I would say Venusaur is better
In-game it really depends on your battle-style
