Hi Steve

You will find numerous oposing opinions on boats mentioned as well as most any topic on sailing ,-
but will try to answer and provide some distance racing insite racing each .
The best thing you could do is sail each ,-then its a matter of boat costs and which type is more actively sailed in your area AND also the type of sailing you prefer .-cruising buoys racing and or distance racing .

The boats -I-20 and 6/0 are very equal ,-they have within 6 sq ft of the same main and jib area ,-the 6/0 has larger jib and has a forward foil of course . The 6/0 has no sq. top main like the I-20 so has a lower C E {center of effort}on the sail plan which helps it in higher wind strengths -The larger 348 type spin for the 6/0 required longer 18 in wire leads to the foil to provide spin pole clearance ,-some good snuffers have been adapted to the longer 6/0 spin pole of up to 14 ft --This as compared to the 270 sq ft spin of the I -20 sometimes used for distance racing with 12 pole and tramp spin bag ,-or buoys racing with cf snuffer at pole end which causes too much drag upwind ,--{tryed it 2 legs this year and changed to pole }.

The 6/0 hull is finer forward in sections and more rounded to reduce form drag and wetted surface area and is better and more easily driven through chop and waves .
The crossbeam -mast is placed well aft on the 6/0 and much more forward on the I-20 .

Both designs could have been much better if the original designer had been directly involved ,
explaination:>
The 6/0 originated when R S stretched out a N 5/8 USING EXISTING MOLDS FOR CONST.and adding to the bows and stern ,

As a result the 6/0 has continueously gone through its mainy different forms and sailplan configurations - original version -express ,-W-1000 versions - N E version etc ,-never quite right due to being locked into the N- 5/8 hull board mast locations that should have been proportionately relocated in balance in a redesign process along with spin adaptation .

The I-20 strangley went through the same error in development ,-its basically an I-18 stretched out ,rather than being specifically redesigned as a 20 .
I understand that the I-20 if left to the original designer would have looked more like the CFR 20 ,-a little better design .

The I-20 is great but could be better .
Its lighter than the 6/0 by an av.28 lbs
the best aspect is its lightweight C F mast
this gives it a performance advantage and is much easier to right and step .
The achilies heel of the I 20 IS THE RUDDERS AND CASTINGS which again being similar to the 18 are not strong enough for more extreme surf conditions per JENSON bch example in the W-1000 2 years ago where we broke 3 sets trying to get out through surf and numerous teams suffered similar boat damage and carnage throughout the race to rudder systems and the lighter boards on the I-20 .
The only way we finished the W-1000 last year was Daves exellent repair skills and us always carrying an extra rudder and casting with us --{hardly a weight saving } -
The last leg in 02 we spent an hour on the beach repairing replacing a rudder while the rest of the fleet sped away at 25 into future storms and 50 mph winds on the nose .

In all fairness the 6/0 has had its problems as well in extreme distance racing conditions -
in 98 we were hit by a micro burst which accellerated the boat from o to 25 in about 6 seconds , when I saw the bows about 6 ft below I knew we were gone ,a second later we went over the bows HARD recall the crew flying through the air landing about 40 ft away . The 6/0 hull inploded from the force splitting on the top -bottom hull seem as these hull are built this way . We repaired it by the next days start but had numerous other problems ,--The back crossbeam bolts popp out of the 6/0 due to 450 sq ft spin loads ,-the only way we made it was to use a trap line and sinch it down to the back crossbeam and nurse the boat in .

Extreme conditions in these distance races call for improved systems and special modification .

An ideal boat for distance racing may be a 6/0 platform or similar more seakindly 20 ft hull form like a S C --then place the lightweight CF mast and I-20 type rig on it -Add an inproved snuffer pole system ,-and used a strong rudder system and reinforced boards. Add a main reef system as the H T had this year and WE have a superior -faster SAFER distance racing 20 ,--Unfortunately these safety features are not allowed under {mfg " class" rules which event organizers use .

The best solution is a Formula 20 rules outline that would allow any standard class 20 ft 8.5 beam boat to race or any combination as descibed to race all within F-20 specifications and rules outline .

Better faster SAFER boats for these extreme races -
In the best interests of all concerned -
They are outlined on the F-20 forum here on Catsailor .

Hope this helps .
Carl