The combat ability of both BL and Sci are the same. The difference is in their support skills. Ressurrection Vs. Turbo Boost.

Ressurrection is a very useful skill especially in tank heavy groups. A ressurrected Mercenary or Assaulter at a key point can change the flow of battle. Also with Elan Plateau, Ressurrectors will be in more demand especially early on as players try to figured out how to conquer the area and bite off much more than they can chew. Unfortunately Ressurrection is the only ability that's useful to a Scientist.
Gauge Up isn't very useful. If your tank is getting hit to the point where he needs his gauge restored it usually means there's 20 players pounding on him and that's a bad situation in itself.

Turbo Boost is the main reason for going Battle Leader. Speed can be a significant advantage in PvP. You can't kill what you can't hit. In mass combat it's not significant, but it shines during small group combat. Chasing down chip runners, picking off stragglers, harrassing reinforcements, taking out scouts, ect.... The opposite is true too, running for you life, moving your group away from a counter attack, getting from point A to point B, ect...
Guage Down is marginally useful. I mainly use it on MAU and Shield Tanks. It's makes taking them down easier, but nothing a little bit more DPS wouldn't have accomplished.
Stone Statue is a useful skill. It's one I should be using more, but often forget I have. It's similar to the "Fake Death" skill. You basically cannot be attacked and take damage. You can still be targetted, but can't be damaged. It allows you a chance to escape getting killed when you're the only one left in your group alive. It's also useful for escaping mobs as they will lose aggro on you. When the timer runs out you have the ability to portal scroll out or run away from the situation. A well timed attack though can still keep you from porting, but the timing on that is rough.

As for me I chose being a Battle Leader for the speed. Hit and Run is my specialty and it's a perfect compliment to my brother's Striker. We hit groups fast and hard and before the reinforcements come in we're already gone. Half the time we've moved into position already to flank the incoming reinforcements
Scientists would compliment a Tank group or suicide (Dementor) sqaud. Bringing back that Assaulter your opponents thought they downed could off balance the group and turn a potential defeat into a victory.

Hopefully this has given you more insight. Accretia could use more of both, it mainly depends on your style of play.

OgamiIttou
lvl 45 Battle Leader
Water Server

Melee: 81
Range: 78
Launcher: 42
Shield: 81
Defense: 76