Really it depends on the requirements of your team, but personally I prefer Arcanine
Scyther: Regarding defense, neither of the two are superb, but Scyther has shortcomings. It's rather frail, though this can be somewhat fixed with eviolite. However,the real issue is its whopping five weaknesses, most notably to the common electric, ice, fire and a painful double weakness to rock, that really stinks when stealth rock is on the field.
Arcanine: Arcanine isn't a defensive behemoth, but it has some defensive merits. It has access to the intimidate ability, which makes it easier for it to take on physical attackers, which is further augmented by the status move Will-o-Wisp. It can also utiliize sunny day to boost its fire moves and lessen it's water weakness, and it's three weaknesses can be handled by a grass type.
Scyther: Scyther is actually quite good offensively. Base 105 atk and 110 spd make it a good physical sweeper. It's movepool is somewhat limited, but thanks to technician, it is usable. With a boosted bug bite and aerial ace for STAB, brick break for coverage, and swords dance for boosting, Scyther is quite a formidable opponent if used right.
Arcanine: Arcanine's diverse movepool, base 110 atk, 100 sp atk and 95 speed means it can be a physical, special, or mixed attacker. Powerful moves such as flare blitz, outrage, solarbeam, wild charge and the prioritized extreme speed make it unpredictable. Flash Fire and justified can power up its moves, and it can use sunny day to abuse a powered up STAB and solarbeam. It even gets cool moves like crunch and close combat through breeding.
Basically, in my opinion the Pokemon Officer Jenny uses is superior, but both have their merits. Hope this helps.