Well, despite both being powerhouses in their respective tiers competitively (Sceptile RU and Shiftry NU in gen 5) all in all, Sceptile outclasses Shiftry in many ways. Sceptile sports higher speed, higher special attack, better bulk and basically has better stats than Shiftry in all but attack and HP. Because of this the only thing Shiftry can do a little better than Sceptile is mix sweeping because it can easily run a defense lowering nature but an offensive boosting one like Hasty/Naive etc seeing as its defenses are worse than terrible. Sceptile also outclasses Shiftry movepool-wise as it has access to moves of many typing not belonging to it's own simple grass, like flying, rock, fighting, dragon, electric, etc. Because of this, Sceptile combined with unburden can be a force very hard to stop. With it's high special attack and blistering speed after unburden it often considered a better sweeper or late-game sweeper. However it does misses out on Nasty Plot which would otherwise probably make it the scariest grass type in nigh all tiers. However due to it still having a very decent attack stat as well as access to Swords Dance, Sceptile technically outclasses Shiftry, also type-wise.
Shiftry has a rather nice offensive dual typing in Grass and Dark which hits everything for neutral damage barring steel types in gen 5. Contrary to Sceptile, Shiftry is a better mixed sweeper as it has superior mixed stats. 100/90 offensive is pretty nice. The thing about Shiftry is that it is a very deadly late-game sweeper in it's tier. With access to Sucker Punch and Swords Dance, this Wind-Demon will be killing many things with just Sucker Punch. What makes Shiftry even deadlier is that it has perfect coverage, just like Sceptile technically also has with access to Rock Slide and Drain Punch as Shiftry has with to Rock Slide as well, as well as Focus Blast, Low Kick and Brick break, depending on its set. Shiftry has a superior offensive typing however defensively, its typing is rather horrid as it does nothing but add weakness upon weaknesses. The only thing it does take care of is an immunity to Psychic types and a resistance to both Dark and Ghost, making Shiftry quite a nightmare for the psychic types in its tier. But with this, it gains a weakness to fighting and a x4 weakness to bug added to its already existing ice, fire and flying weakness.
TL;DR: All in all, the suprior grass type is Sceptile.