I don't know why this isn't a thing yet, but it should be, so I made it.
These are the two (as of this writing) Grass Starter Mega Evolutions.
Mega Venusaur is a defensive behemoth (single best defensive Mega in the game as of right now) with only two fairly uncommon weaknesses and an impressive array of resistances (Water, Electric, Fighting, Grass, and now Fairy.) It's ability to recover HP in a myriad of ways (Synthesis, Leech Seed, Giga Drain) puts it light-years ahead of every other defensive Mega in the game, and on par with the toughest defensive threats this side of Ubers. It can shut down Fairies with a well placed Sludge Bomb, doesn't fear Ice Beams from Water Types, and rolls its eyes at your Infernape's pathetic attempts to Flare Blitz it.
Mega Sceptile, for its part, gains a Dragon typing. This means that it can now fire off STAB Dragon Claws, Dragon Pulse, and Outrage, and that it gets impressive neutral coverage, only walled by Steel types, Togekiss, and Whimsicott (but hey, that's what coverage moves are for.) It's ability, Lightningrod, is comparatively less useful than Venusaur's Thick Fat because Sceptile already 4x resists Electric moves due to its typing, but if you look a little deeper, you can see some hidden uses for it. For instance, it is now immune to Thunder Wave (and all Electric forms of paralysis.) In Doubles, it can work well with an Electric Type partner and get a boost from what would otherwise be splash damage. It works on Rain Teams as a check to Grass and Electric types that threaten your Water types, etc. Its typing also provides it with a handy 4x resist to both Grass and Water, and it loses its weakness to Fire. It's not all good, however. The trade-off is two added weaknesses in Dragon and Fairy, and a 4x weakness to Ice.
Its stats are still offensive in nature, with an incredibly buffed up special attack and speed stat and a usable attack stat, meaning it can be a pretty decent mixed sweeper (provided it gets the right movepool.)
So, which of these forest titans is your favorite? Which one would win in a battle? Which one is better designed (both aesthetically and in terms of gameplay?)
My money's on Venusaur, as STAB Sludge Bomb will still hurt Sceptile while the latter can only hit back with neutral Dragon moves or Hidden Power.