It's a bit unclear how far back we want to define "traditional" (which in turn can vary the scope of the answer), but if we define "traditional" to be the slalom boards of the last few seasons (ie : the current "life" of slalom, as compared to the previous "life" of slalom pre extinction) ...
The real difference and advantage would be the increase in rideability and range, which in most conditions makes the new boards faster, or easier to be fast on. The iSonics really have quite a flat trim/ride, which is something that was not so easily achieved on traditional boards. Especially with the current range of iSonics (with even more improved jibe) there really is not any significant performance downside to the new boards.
Upwind ability, ability to efficently
carry bigger sails and fins (where required) early planing, planing thru lulls and carrying power thru jibes all rate better on iS than more traditional shapes.
The traditional narrow boards feel possibly sharper or faster in the water and in some cases give a enhanced speed "sensation" however head to head and gps testing shows the calmer new generation boards are deceptively fast.
It's also worth noting that a poorly designed short/wide slalom could easily be a real ugly board to ride/jibe, however with iSonics the results have been very positive, both to our testing and (maybe more importantly) the general public.
There are very few who have spent time (or money) on the iSonics who don't find them a better overall slalom solution than the longer, narrower traditional designs.
For a 94kg rider in 15-25kts Bay chop, the 3 sizes in typical consideration would be iS94/101/111:
94 would be a good top end board, more demanding and potentially critical around the 15 kts range, especially if the 15kts was marginal. For a serious focus on overall range, the 101 is a really good choice (and my favorite), and it's early planing (with moderate skill and decent tuning) is not far behind the larger 111 and even 122. 101 has a really good jibe and good speed in both light and powered conditions. A bit bouncier over chop in high winds, less of an issue with bigger riders.
iS111 is a safer bet for heavier riders in lower (~15)and mid range(15-20), but with more compromise on the real top end 20+, especially if it gets really lumpy or choppy.(although, by comparison, you will find the iS111 still a better choice in these conditions than most "traditional" longer, narrow designs.
OK, make that 4 options; The iS122 is an absolute wildcard in that windrange, but for a heavy rider remains also a possibility with excellent low end and mid range(10-20kts), more challenging (but still very capable) in serious chop above 20 kts. But an unreal jibe for such a "big" slalom. Doubt it ? Me too. Try it.
Deciding which end of the 15-25 kt range you will spend most of your sailing time in (on this board) - or alternately the sail size range/swetspot you desire)will influence the optimal choice.
Most people initially fear the wider, flatter shape in chop, but a few sessions quickly convert them, and in the clear majority of cases, there is no "going back" after that. If you've any doubts, please try for a demo or test. Or more info.
Cheers ~ Ian