Mmmm Soi Fon's advantage is her speed here.
She could make clones and Shikai Nel.
Nel Tu is mai hot waifu. And Soi Fon would win. Nel is definitely more destructive, but soi Ffon has speed and skill over her. Kinda mad kubo never went more in depth with shunko, when soi fon fought barragan she never once used it, which ticked me off -___-
Soi Fon's primary abilities are her incredible speed as a Shunpo master (being a disciple of the Goddess of Flash, Yoruichi Shihouin) which she can use to not only evade attacks but create 'clones' of herself to confound her opponent, her skill as the current head of the Onmitsukido (or more generally, Covert Ops) makes her out to be a well-practiced assassin, and her Shunko ability which drastically increases her strength and durability to the point of pushing away blades with the bare palm of her hands. The shikai of her zanpakuto, Suzumebachi, would be her preferred method of attack thanks to its ability to kill someone effortlessly should they be struck in the same place twice. We can also take into effect her bankai, Jakuho Raikoben, but not only does that tax her greatly after use, it requires extensive preparation beforehand which makes it unsuited for high-speed combat.
Nel, in her 'original form', actually boasts a lot of the same abilities as Soi Fon by different names. She's an expert hand-to-hand combatant just as Soi Fon is, she's gained mastery over the Arrancars' ability of Sonido which theoretically makes her the fastest of all Arrancar to date, and she actually has a rather high intellect to a point where she's been able to decipher opponents' moves based solely on previous attacks and their temperament. She has the advantage of her Hierro (or 'Iron Skin') which can deflect minor sword blows, but given that's meant to defend against widespread sword slashes and not concentrated thrusts, this may not be as large an advantage as once thought. She also has her Resurreccion which transforms her into a large centaur, thus increasing her strength, spiritual pressure and grants her a new ability with her transformed sword, Lanzador Verde (or 'Jade Lance') in which she throws it forward at a high speed with a heavy spin on it to create a sort of drill-like effect; during transit, the spinning lance collects spiritual energy to further intensify its power to a point where it was not only able to break through Nnoitra's own Resurreccion but his self-proclaimed 'strongest and thickest Hierro', basically killing him in a single blow.
Will it come down to a battle of speed and precision, or will the pair of them flaunt their most powerful abilities in a game of high-stakes chicken? Either way, the destructive force of Jakuho Raikoben's explosion was so great that, even at a considerable great distance of at least a mile, she was blown back by the force of her own attack and reduced whatever the missile hit to mere rubble; and in terms of sheer killing power, Suzumebachi's 'two-hit kill' means she only needs to choose her attacks very carefully and strike twice... though not necessarily in the same place as, in the manga and anime, she was able to pierce right through an enemy's skin from the front and place her killing mark on a character's heart in order to strike at them from the back and pierce the skin in the exact same place, counting it as a two-hit strike in the same area once her blade touched that character's heart again.
If Soi Fon was serious about this battle—and tell me, when isn't she serious about murdering someone in cold blood—she would definitely come out on top, but at least Nel would give her a run for her money... pun not intended.