Breloom and Scizor are both great Pokemon with about the same stats. It's really whatever the purpose you plan to use them for in battle.
Breloom would make a good single-battler, with Toxic Orb and Poison Heal to keep it's HP on top, and a wide variety of moves to keep the opponents on their toes. Put your opponent to sleep, do a swords dance or two and BAM hit them hard with Stone Edge or Focus Punch. Contrary to what blazo said, you should definitely keep Stone Edge, IF you want an offensive Breloom. It offers great coverage to it's weaknesses while having a high attack power. If you want a more defensive Breloom (maybe better for double battles) go with this:
Breloom @ Toxic Orb
Trait: Toxic Heal
EVs: 252 HP / 56 Atk / 200 Spd
- leech seed
- focus punch/drain punch
This set will REALLY mess with your opponents mind, putting them to sleep and then leeching their health in numerous ways. >=) The most common Pokemon to counter your Breloom would probably be Celebi (knows Psychic and Hidden Power Ice; can also counter all your STAB attacks) and this move set would also let you last a LOT longer against him and another common counter, Amoonguss. For a double battle you could have a strong Dragon or Fire type with a wide variety of moves to sweep the opponent after putting them to sleep.
Your Scizor, with Baton Pass to set up for a Mega, is also great for a single battle. It could also Baton Pass for a Dragon type with a wide movepool to rip through the opponent. The first suggestion I'll make is to change the Weakness Policy item to a Life Orb. A good strategy would be to use Scizor as like a half-sweeper: deal out a few moves, and then Baton Pass for your Mega to finish them off. If you want to change your ability to Technician and abuse it then here's a good moveset:
Scizor @ Life Orb
EVs: 252 Atk / 252 HP / 6 Spd
- Bullet Punch
- Bug Bite (steals opponents pesky berries too)
- Aerial Ace/Brick Break
- Baton Pass (if you decide not to use this as a Baton Passer, switch this for Swords Dance)
This can deal some pretty heavy damage and despite it's speed, Scizor can withstand most attacks that go before it. If the opponent sends out something that CAN take care of Scizor, just Baton Pass and use your Mega to take care of it. Now, if you want to use Scizor solely as a defensive tank Baton Passer, use this set:
Scizor @ Leftovers
EVs: 252 HP / 112 Def / 144 Spd
- Iron Defense/Agility
- Baton Pass
This moveset is really flexible, but I think Toxic is a must to go along with Baton Pass. Put a timer on your opponents life span, and then hit em hard with your Mega.
Hope this helps!