Gearing is everything. A LS motor is designed to provide more power and in stock form has some pretty nice upgrades over a stock 350. Better spark, better flowing heads, better induction system, - it's just a higher performance engine right out of the factory. It is built to handle higher RPM's, giving more high end power and losing some torque on the low end as a result. Going stock on both motors, the LS will want some lower gearing in order to better utilize its power advantage - but given that it should easily outperform a stock SBC. Realistically a TBI engine doesn't perform much better than a mid-70's, detuned, emission-laden SBC. The potential is there though - with a decent cam, stroke it, head upgrades, improved induction and a decent exhaust - you can make a pretty good towing machine out of it. Target low end torque and put in some decent gearing and the LS motors would be hard-pressed to match the towing capability. The SBC makes pretty good low end torque - which really helps in towing anything heavy.