IMO - If you are putting a motor in a truggy and are on a budget, spend $150 and get the Sportworks/Dynamite/LOSI .28. Tons more horsepower than either you just mentioned. You will get 7 gallons pretty easy. They are half the cost to put a piston and sleeve in. AND....On the track you will not be out motored. These motors have proven to be the bang for the buck
when there are not motor restriction rules.
When we switched to this motor in our shop truggy we had to gear it up and soften the clutch. It had tons of rip with a 7mm restrictor and would flat out run all but the most radical of buggies down the 180 ft staight. The only thing we had to worry about was over jumping everything. Super Fast!
BTW - The motors you had mentioned do not make good power on the low end. S3 Might be OK for a buggy on a slick track and the S5 is just a little better. Good motors and I do have both of them. You might want to do a little more looking when it comes to truggys.