Since so many of those w/whom the Stooges worked were veterans of the Sennett studio (Del Lord, most importantly, but also Vernon Dent, Chester Conklin, and others) I don't think there'd be too much of a perceptible difference between what we know as the Stooges style and Sennett's style, save that you might've ended up with more large-scale "rallies" e.g. as seen in FALSE ALARMS. The Stooges' dominant style, at least through the Curly era, is an amalgamation of Sennett w/vaudevillian dialogue. Sennett's gag-for-gags'-sake format is pretty evident throughout the Stooge canon. So does "scare" comedy, or comedy dependent upon a semi-melodramatic premise: these elements predominate right through the Bessers.
BTW, the "Keystone" sobriquet was phased out by, I think, 1918 or so; Sennett's studio was simply "Mack Sennett Comedies" after then.
Roach is a different question; you'd find more comic set-pieces: longer, more elaborate gag sequences. Two of the Stooge shorts on which Charley Chase worked seem very close to Roach's style, TASSELS IN THE AIR and TERMITES OF 1938. SLIPPERY SILKS and POP GOES THE EASEL and, to some extent, THREE LITTLE BEERS, all of which rely as much on sight gags as verbal humor, are pretty close to Roach's mature style. BEERS in particular is basically just a string of gags built around a particular setting, the kind of near-ad lib format used by L&H. Roach's shorts also used whimsical, semi-surreal gags which crop up in the Chase-influenced Stooge shorts, such as Larry and Curly playing checkers w/paint cans on a block tiled floor, or the elaborate mousetrap in TERMITES OF 1938 (based on a similar gag in Chase's PUBLIC GHOST NO. 1). Chase's typical "comedy of embarrassment" also surfaces in both these films. Roach himself had a dislike for gags involving black humor - you'd never find a comedy like THEY STOOGE TO CONGA at Roach, the Boss would've nixed it. However, Roach would've permitted the team much more leeway in crafting their own comedy, given them longer shooting schedules and encouraged a more improvisatory attitude.
However, in a way, these are moot questions: the Stooges came along at a time when both Sennett and Roach were ending production of shorts. Sennett's studio went into bankruptcy in 1933, which is why so many Sennett veterans ended up at Columbia, which was just developing a comedy unit. Roach had phased out 2-reelers by the end of the 35-36 season, and discontinued shorts altogether by the end of the 37-38 season. Would the Stooges have been as efefctive if they'd gone into films in the 20s? Hard to say, given the fact it would've taken some years to develop the flawless teamwork and timing they did display by 1933.
