The Renault engine is much more durable. I know a guy who lives by me that has had an S1a since the early '70s and he has put over 300,000 miles on it. Of course, he is a british car mechanic and probably rebuilt it numerous times. The Renault engines can be built to twin cam levels of horsepower. The S1 and S2s are a little bit more stable at speed. Colin Chapman fought cutting down the sails for the new Twin Cam shape because those sails are meant to be aerodynamic stabilizers. An S2 in most cases will be significantly lighter than a Twin Cam or a Special (though my TCS is close to S2 weight now and I plan on taking another 40 lbs off). Early S2s have the look of an S1 but less of the headaches). Type 54 Fed spec models have the lowered front end and suspension of the ROW S2s but have the larger Renault motor. But that being said, the Twin Cam motor has a true and long racing heritage and sounds exquisite. It is a work of art and an amazing automotive design. One of the all time great 4 cyl motors.