Shit... Xtasy can make some really difficult to call matches sometimes. I guess it's time for another analysis.
First off is their martial skill. Aryn Leneer's lightsaber form is never specified, but given her role as a Consular, her force abilities, her speed and agility, and her ability to dual wield, I'd speculate she studied Ataru and Niman. Her skills were such that she was capable of competing with Darth Malgus in a lightsaber duel, even managing to land a few hits on the Sith Lord. Ataru and Niman compliment eachother quite nicely when you consider that they take advantage of physicality and force abilities, while providing a lot of general versatility and pretty fleshed out dual blade training.
Asajj Ventress was a Master of Jar'kai, and supplemented this style with Makashi. Her skill was such that she was capable of defeating members of the Jedi council, but not at the point to which she was capable of besting her master, Darth Tyranus. Jar'kai and Makashi also compliment eachother quite nicely when you consider the advantages of energy efficiency and precision with two blades.
This is actually an interesting contest to call, both wield highly effective styles of lightsaber combat, and both are skilled enough to challenge high level opponents. Aryn did far better against Malgus than Ventress did against Tyranus and I consider Tyranus and Malgus to be roughly even as duelists, however it is SWTOR Malgus that I consider to be on Tyranus's level as a duelist, and Aryn fought Malgus in Deceived. This being said, Ventress's defeat of various council members and Grievous lead me to believe that Ventress is the superior duelist of the two. I give Asajj Ventress the edge in martial skill.
Next is their physical capabilities. Aryn Leneer was fast enough to appear to be in multiple places at once, dash a hundred meters, be an almost impossible to trace blur for an ex-commando's eye to follow, and make it seem as if time had slowed down. Her strength was such that she could meet Malgus's blows in combat, though it made her arms quiver. Her ability to take injury was also quite impressive, as she was able to press on after Malgus threw her into stuff and knocked the wind out of her.
Asajj Ventress was very fast, most notably being able to match Grievous blow for blow on Dathomir near the end of their fight. Her strength was such that she was capable of combating the likes of Anakin Skywalker, and Savage Opress, though both have overwhelmed her with physical strength before. Her ability to take injury was fairly impressive as well, as she was capable of recovering from Savage's physical blows, and recovering from Tyranus's lightning quickly enough to save herself and her fellow Nightsisters.
The two seem quite similar physically. Based on speed feats similar to Aryn's that I am familiar with, her speed would seem to be approaching ten strikes per second in lightsaber combat, though maybe a bit less. Ventress was capable of matching Grievous's blows, and Grievous's max shown against Obi-wan was twenty strikes per second, however by this point Grievous was only attacking with one arm at a time, unlike he was against Obi-wan. Ventress may have also been aided by the connection the Nightsisters have with Dathomir. Aryn's strike per second speed seems slightly superior to me, as well as her other speed feats being beyond what Ventress has shown, so I'd say Aryn is the faster of the two. As per physical strength, Aryn wasn't overwhelmed as much as Ventress was, however Aryn also didn't meet power blows head on as much as Ventress did, so their strength is fairly challenging to judge. If I had to choose however, I'd give Ventress the edge in this regard. Their ability to take injury also seems fairly even with perhaps Ventress having a slight edge due to her performance against Tyranus's lightning. I have given Ventress more physical edges than I have Aryn, however the edge I have given Aryn is by a larger margin and is more significant in this contest, so I'd give the physical edge to Aryn Leneer.
Now for their force abilities. Aryn Leneer was a very powerful telekinetic, capable of blasting sith off of their feat with enough force to shatter statues, capable of redirecting powerful telekinetic waves, capable of throwing large objects around, talented at saber throws, and capable of overpowering Malgus’s TK grip on her lightsaber. She was also skilled enough at tutaminus to dissipate Malgus’s lightning once.
Asajj Ventress was also a potent telekinetic, being capable of collapsing large portions of a cave on Rattatak, and frequently applying force grip. At one point she even held Anakin and Obi-wan in a force choke at the same time, though she did catch them off guard and was in an intense rage from her betrayal at the hands of her master. Ventress was also capable of a Nightsister blood trail.
Both seem to be very potent telekinetics, though Aryn’s more consistent, more frequent, and more potent telekinetic displays impress me more. As such Aryn strikes me as the more impressive telekinetic. As per their other force abilities, neither will really play a role here as a blood trail has no use in combat, and neither use any energy based attacks for tutaminus to be useful. With Aryn edging it out in the one area that actually matters, I’d give Aryn the edge as a force wielder.
Overall, this was a tough one to call, but I’d say Aryn has a larger advantage in the force than Ventress does in physical combat. For these reasons, I declare Aryn Leneer the winner!