Ron K does make the excellent point about anti roll bar size versus spring rates. Increasing either will reduce vehicle roll in corners so if your objective is just reduced roll they are both options and his advice about not doing both at the same time is wise. If your hang-up is just vehicle body roll then the larger ARB might be the better solution as it will not have quite the same negative impact on suspension compliance over things like speed bumps / rumble strips that high spring rates will have. Also, if you leave your vehicle at the original ride height it does not mess with the camber and caster values on the car and you don't have to address the rear axle offset and propeller shaft / U joint alignment problems that may result from a significant drop in vehicle height.
As an observation, vehicle roll is not necessarily a bad thing. Reducing roll may make you feel more comfortable; but, reducing roll can reduce camber and weight transfer which may actually reduce tire grip. When you put a larger front ARB on a car, it will reduce vehicle roll; but, you will in general find that it changes the balance, increasing under steer. In the previous century; big marshmallow North American sedans were noted for really soft springs and really fat front ARBs and a healthy dose of understeer. In the seventies IPD marketed bigger ARBs for vintage Volvos selling both front and rear ARBs. The front ARBs definitely reduced body roll. They also increased under steer because of reduced grip in the front. Some years later IPD started marketing the ARBs just as a front and rear ARB package. The rear ARB reduced grip at the back which added a little over steer which helped balance out the understeer associated with the large front ARB returning the car to more neutral steer; but, not necessarily increasing overall grip. Increased ARB stiffness reduces both camber change and rear axle articulation which generally reduces tire effectiveness.
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