[reachzero]

First of all, let me weigh in on Grass/Dragon. Grass/Dragon is a terrible idea for this CAP. The greatest problems for Sun are unequivocally Latias and Heatran. Grass/Dragon does not help with either problem. It is true that Sun teams do need a Water resist and a Ground resist. However, there are already Pokemon that fill this need as well as CAP5 ever could: Latias and Dragonite. There is no comparable Pokemon that can reliably take on Latias and Heatran for a Sun team. In fact, Grass/Dragon would actually invite other teams to use Heatran and Latias, which would only make problems worse for Sun teams. A Grass/Dragon Pokemon would do more to hurt Sun than help it.

Dark/Electric is not a terrible combination, but it is noticeably worse than Dark by itself. Compared to Dark by itself, Dark/Electric becomes weak to Ground, while gaining only one useful resistance, Electric, alongside relatively useless resistances in Steel and Flying. Steel is only useful in turns of Scizor and Ferrothorn; neither of which are particular problems for a Sun team (which by definition has Ninetales) and which both have nasty options that aren't Bullet Punch or Gyro Ball to hit CAP5 with anyway. Flying is only for Hurricane, and let's face it, with Tornadus-t gone, Hurricane is virtually a non issue. The only Hurricane users of any substance are Dragonite and Tornadus, and Tornadus is not much of a factor anyway. To give you an idea, the last month before the arrival of Tornadus-t with BW2 was June 2012. Tornadus was #36, just more common than Conkeldurr and four slots below Donphan. Hurricane is no longer a major factor in the mainstream OU metagame. In return, the Ground weakness is a considerable liability against numerous Pokemon, notably Landorus, one of the worst Pokemon to give a free turn, for any team but particularly a Sun team. Dark and Electric also synergize very poorly as STABs, since the value of being Dark type offensively comes on the physical side (Pursuit, Sucker Punch, Crunch) while Electric has very poor physical moves (Wild Charge, basically; Fusion Bolt, Bolt Strike and Volt Tackle are all signature moves, and for the most part inaccessible to us).

Grass/Electric shares some of the worst characteristics of Grass/Dragon and Dark/Electric; it cannot beat Latias, it has only one useful resistance, it's STABs are largely redundant and don't synergize well. The largest benefit of Grass types is being resistant to both Water and Ground; being neutral to Ground does much to hurt a Grass type's usefulness (and Venusaur can attest. It's pretty hard to actually get in safely!). It is pretty telling that there is only one Grass/Electric, but it has good stats and a decent movepool, and is RU.

By comparison, Dark/Grass resists both Ground and Water, consistently beats Latias (assuming the right stats, as all of these typings do), can switch in on Politoed and Hippowdon freely (probably not Tyranitar except very carefully). Grass typing means that Terrakion and Keldeo need to think twice before switching in, which really helps a Dark Pokemon. While it is true that Venusaur is also a Grass type, this actually works in both Pokemons' favor somewhat in that it relieves Venusaur's four moveslot syndrome--if Lati@s is dealt with, Venusaur can afford to use a Grass move, Nature Power/Earthquake and HP Fire rather than Sludge Bomb, which helps Venusaur considerably.

Dark/Grass is a very adequate typing for a defensive pivot for a Sun team, while boasting exactly the right sort of firepower to take down Latias, one of the largest threats to a Sun team. It is hands-down the best type for CAP5.