The choice between Treeko or Mudkip really depends on the Physical/Special attacking capabilities of the rest of your team, as well as whether or not you're going to use their Mega forms or not. For that reason, I will give reasons for both the base formes and their Megas.
Swampert
- Swampert is a complete powerhouse physically
- Also has decent bulk, with 100/90/90 defenses
- Has an amazing movepool
- Has access to five HMs
- Electric immunity
- While Swampert has only one weakness, it is a glaring one
- Special Attack leaves something to be desired
- Speed is downright awful
Mudkip/Swampert should be used if tour team is lacking physically. He has wonderful Attack and the ability to kick major butt, as well as tank hits with is high HP stat. Swampert posesses one immunity and one weakness. He is good for a team that counters Grass heavily (mostly Fire/Flying/Bug) or a team that needs help handling Electric types. Swamp also gets access to a wide variety of coverage moves. The best item for Swampert would probably be either Leftovers or Expert Belt. Also, go for an Admamant Nature (raises Attack and lowers his useless Special Attack). Suggested set (assuming no EVs):
Swampert@ Leftovers/Expert Belt
Ability: Torrent
Nature: Adamant
- Earthquake (TM, use Mud Shot until you get this)
- Waterfall (HM, strong Water attack)
- Ice Punch (Move Tutor, covers Ice weakness)
- Hammer Arm/Brick Break/Rock Slide (your choice of coverage moves)
Mega Swampert
- Out of this world Attack
- Swift Swim makes him REALLY fast
- Half-decent Special Attack
- Crazy bulk with 100/110/110 defenses
- Still has a glaring Grass weakness
Mega Swampert is good when used on teams with Pokémon that take advantage of the Rain, such as Ludicolo. With Mega Swampert's Nature, either keep Adamant for more power or Jolly for more Speed. This is the recommended set:
Swampert@ Swampertite
Ability: Torrent -> Swift Swim
Nature: Adamant/Jolly (power or speed)
- Earthquake
- Waterfall
- Ice Punch
- Rain Dance (TM, sets up Rain for the Swift Swim speed boost)
Sceptile
- Glorious Speed
- High Special Attack
- Access to a wide movepool
- Many weaknesses
- Shoddy defenses
Treecko/Sceptile is a fabulous Grass-type, able to run a strong Special, Physical or Mixed set. He is best used on teams that need speedy and powerful attacks. For Items, definitely use Big Root to increase HP recovered by Giga Drain. For a Nature, use Hasty. Hasty boosts Speed while not taking away from either attacking stat, reducing its already-bad Defence. In-game, it's okay to run Mixed sets, so try this:
Sceptile@ Big Root
Ability: Overgrow
Nature: Hasty
- Giga Drain (recovery, only obtainable by waiting until Lv. 21 to evolve Treecko)
- Leaf Blade (strong Physical move)
- Drain Punch (Move Tutor, More recovery, coverage)
- Rock Slide (covers Ice and Flying weakness)
Mega Sceptile
- Stellar Speed and Special Attack stats
- Usable Attack stat
- Usable Ability
- Glaring Ice weakness
- Added Fairy and Dragon weakness
Mega Sceptile is an amazing Special Sweeper, brilliant at crushing teams under its boot. Just be sure to remove all Fairy and Ice types, as Mega Sceptile gas few reliable ways of protecting itself from that. It compliments Water and Flying teammates due to Lightningrod giving it Electric immunity and a possible Special Attack boost. For Mega Sceptile, take advantage of its fantastic Special Attack and run an all-Special moveset:
Sceptile@ Sceptilite
Ability: Overgrow ->Lightningrod
Nature: Hasty
- Giga Drain
- Dragon Claw/Dragon Pulse (TM/move tutor, STAB)
- Earthquake
- Drain Punch/Pursuit/X-Scissor/Rock Slide (your choice of coverage)
I hope you can glean some useful information from my answer, and have fun!
